Rollup merge of #109475 - scottmcm:simpler-shifts, r=WaffleLapkin

Simpler checked shifts in MIR building

Doing masking to check unsigned shift amounts is overcomplicated; just comparing the shift directly saves a statement and a temporary, as well as is much easier to read as a human.  And shifting by unsigned is the canonical case -- notably, all the library shifting methods (that don't support every type) take shift RHSs as `u32` -- so we might as well make that simpler since it's easy to do so.

This PR also changes *signed* shift amounts to `IntToInt` casts and then uses the same check as for unsigned.  The bit-masking is a nice trick, but for example LLVM actually canonicalizes it to an unsigned comparison anyway <https://rust.godbolt.org/z/8h59fMGT4> so I don't think it's worth the effort and the extra `Constant`.  (If MIR's `assert` was `assert_nz` then the masking might make sense, but when the `!=` uses another statement I think the comparison is better.)

To review, I suggest looking at https://github.com/rust-lang/rust/pull/109475/commits/2ee0468c49be9f8ea68b7b935ea1f4c2555849e9 first -- that's the interesting code change and has a MIR diff.

My favourite part of the diff:
```diff
-        _20 = BitAnd(_19, const 340282366920938463463374607431768211448_u128); // scope 0 at $DIR/shifts.rs:+2:34: +2:44
-        _21 = Ne(move _20, const 0_u128); // scope 0 at $DIR/shifts.rs:+2:34: +2:44
-        assert(!move _21, "attempt to shift right by `{}`, which would overflow", _19) -> [success: bb3, unwind: bb7]; // scope 0 at $DIR/shifts.rs:+2:34: +2:44
+        _18 = Lt(_17, const 8_u128);     // scope 0 at $DIR/shifts.rs:+2:34: +2:44
+        assert(move _18, "attempt to shift right by `{}`, which would overflow", _17) -> [success: bb3, unwind: bb7]; // scope 0 at $DIR/shifts.rs:+2:34: +2:44
```

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