Rollup merge of #59283 - SimonSapin:branchless-ascii-case, r=joshtriplett

Make ASCII case conversions more than 4× faster

Reformatted output of `./x.py bench src/libcore --test-args ascii` below. The `libcore` benchmark calls `[u8]::make_ascii_lowercase`. `lookup` has code (effectively) identical to that before this PR, and ~~`branchless`~~ `mask_shifted_bool_match_range` after this PR.

~~See [code comments](https://github.com/rust-lang/rust/pull/59283/commits/ce933f77c865a15670855ac5941fe200752b739f#diff-01076f91a26400b2db49663d787c2576R3796) in `u8::to_ascii_uppercase` in `src/libcore/num/mod.rs` for an explanation of the branchless algorithm.~~

**Update:** the algorithm was simplified while keeping the performance. See `branchless` v.s. `mask_shifted_bool_match_range` benchmarks.

Credits to @raphlinus for the idea in https://twitter.com/raphlinus/status/1107654782544736261, which extends this algorithm to “fake SIMD” on `u32` to convert four bytes at a time. The `fake_simd_u32` benchmarks implements this with [`let (before, aligned, after) = bytes.align_to_mut::<u32>()`](https://doc.rust-lang.org/std/primitive.slice.html#method.align_to_mut). Note however that this is buggy when addition carries/overflows into the next byte (which does not happen if the input is known to be ASCII).

This could be fixed (to optimize `[u8]::make_ascii_lowercase` and `[u8]::make_ascii_uppercase` in `src/libcore/slice/mod.rs`) either with some more bitwise trickery that I didn’t quite figure out, or by using “real” SIMD intrinsics for byte-wise addition. I did not pursue this however because the current (incorrect) fake SIMD algorithm is only marginally faster than the one-byte-at-a-time branchless algorithm. This is because LLVM auto-vectorizes the latter, as can be seen on https://rust.godbolt.org/z/anKtbR.

Benchmark results on Linux x64 with Intel i7-7700K: (updated from https://github.com/rust-lang/rust/pull/59283#issuecomment-474146863)

```rust
6830 bytes string:

alloc_only                          ... bench:    112 ns/iter (+/- 0) = 62410 MB/s
black_box_read_each_byte            ... bench:  1,733 ns/iter (+/- 8) = 4033 MB/s
lookup_table                        ... bench:  1,766 ns/iter (+/- 11) = 3958 MB/s
branch_and_subtract                 ... bench:    417 ns/iter (+/- 1) = 16762 MB/s
branch_and_mask                     ... bench:    401 ns/iter (+/- 1) = 17431 MB/s
branchless                          ... bench:    365 ns/iter (+/- 0) = 19150 MB/s
libcore                             ... bench:    367 ns/iter (+/- 1) = 19046 MB/s
fake_simd_u32                       ... bench:    361 ns/iter (+/- 2) = 19362 MB/s
fake_simd_u64                       ... bench:    361 ns/iter (+/- 1) = 19362 MB/s
mask_mult_bool_branchy_lookup_table ... bench:  6,309 ns/iter (+/- 19) = 1107 MB/s
mask_mult_bool_lookup_table         ... bench:  4,183 ns/iter (+/- 29) = 1671 MB/s
mask_mult_bool_match_range          ... bench:    339 ns/iter (+/- 0) = 20619 MB/s
mask_shifted_bool_match_range       ... bench:    339 ns/iter (+/- 1) = 20619 MB/s

32 bytes string:

alloc_only                          ... bench:     15 ns/iter (+/- 0) = 2133 MB/s
black_box_read_each_byte            ... bench:     29 ns/iter (+/- 0) = 1103 MB/s
lookup_table                        ... bench:     24 ns/iter (+/- 4) = 1333 MB/s
branch_and_subtract                 ... bench:     16 ns/iter (+/- 0) = 2000 MB/s
branch_and_mask                     ... bench:     16 ns/iter (+/- 0) = 2000 MB/s
branchless                          ... bench:     16 ns/iter (+/- 0) = 2000 MB/s
libcore                             ... bench:     15 ns/iter (+/- 0) = 2133 MB/s
fake_simd_u32                       ... bench:     17 ns/iter (+/- 0) = 1882 MB/s
fake_simd_u64                       ... bench:     16 ns/iter (+/- 0) = 2000 MB/s
mask_mult_bool_branchy_lookup_table ... bench:     42 ns/iter (+/- 0) = 761 MB/s
mask_mult_bool_lookup_table         ... bench:     35 ns/iter (+/- 0) = 914 MB/s
mask_mult_bool_match_range          ... bench:     16 ns/iter (+/- 0) = 2000 MB/s
mask_shifted_bool_match_range       ... bench:     16 ns/iter (+/- 0) = 2000 MB/s

7 bytes string:

alloc_only                          ... bench:     14 ns/iter (+/- 0) = 500 MB/s
black_box_read_each_byte            ... bench:     22 ns/iter (+/- 0) = 318 MB/s
lookup_table                        ... bench:     16 ns/iter (+/- 0) = 437 MB/s
branch_and_subtract                 ... bench:     16 ns/iter (+/- 0) = 437 MB/s
branch_and_mask                     ... bench:     16 ns/iter (+/- 0) = 437 MB/s
branchless                          ... bench:     19 ns/iter (+/- 0) = 368 MB/s
libcore                             ... bench:     20 ns/iter (+/- 0) = 350 MB/s
fake_simd_u32                       ... bench:     18 ns/iter (+/- 0) = 388 MB/s
fake_simd_u64                       ... bench:     21 ns/iter (+/- 0) = 333 MB/s
mask_mult_bool_branchy_lookup_table ... bench:     20 ns/iter (+/- 0) = 350 MB/s
mask_mult_bool_lookup_table         ... bench:     19 ns/iter (+/- 0) = 368 MB/s
mask_mult_bool_match_range          ... bench:     19 ns/iter (+/- 0) = 368 MB/s
mask_shifted_bool_match_range       ... bench:     19 ns/iter (+/- 0) = 368 MB/s
```

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