Rollup merge of #36917 - nnethercote:speed-up-plug_leaks, r=eddyb

Speed up `plug_leaks`

Profiling shows that `plug_leaks` and the functions it calls are hot on some benchmarks. It's very common that `skol_map` is empty in this function, and we can specialize `plug_leaks` in that case for some big speed-ups.

The PR has two commits. I'm fairly confident that the first one is correct -- I traced through the code to confirm that the `fold_regions` and `pop_skolemized` calls are no-ops when `skol_map` is empty, and I also temporarily added an assertion to check that `result` ends up having the same value as `value` in that case. This commit is responsible for most of the improvement.

I'm less confident about the second commit. The call to `resolve_type_vars_is_possible` can change `value` when `skol_map` is empty... but testing suggests that it doesn't matter if the call is
omitted.

So, please check both patches carefully, especially the second one!

Here are the speed-ups for the first commit alone.

stage1 compiler (built with old rustc, using glibc malloc), doing debug builds:
```
futures-rs-test  4.710s vs  4.538s --> 1.038x faster (variance: 1.009x, 1.005x)
issue-32062-equ  0.415s vs  0.368s --> 1.129x faster (variance: 1.009x, 1.010x)
issue-32278-big  1.884s vs  1.808s --> 1.042x faster (variance: 1.020x, 1.017x)
jld-day15-parse  1.907s vs  1.668s --> 1.143x faster (variance: 1.011x, 1.007x)
piston-image-0. 13.024s vs 12.421s --> 1.049x faster (variance: 1.004x, 1.012x)
rust-encoding-0  3.335s vs  3.276s --> 1.018x faster (variance: 1.021x, 1.028x)
```
stage2 compiler (built with new rustc, using jemalloc), doing debug builds:
```
futures-rs-test  4.167s vs  4.065s --> 1.025x faster (variance: 1.006x, 1.018x)
issue-32062-equ  0.383s vs  0.343s --> 1.118x faster (variance: 1.012x, 1.016x)
issue-32278-big  1.680s vs  1.621s --> 1.036x faster (variance: 1.007x, 1.007x)
jld-day15-parse  1.671s vs  1.478s --> 1.131x faster (variance: 1.016x, 1.004x)
piston-image-0. 11.336s vs 10.852s --> 1.045x faster (variance: 1.003x, 1.006x)
rust-encoding-0  3.036s vs  2.971s --> 1.022x faster (variance: 1.030x, 1.032x)
```
I've omitted the benchmarks for which the change was negligible.

And here are the speed-ups for the first and second commit in combination.

stage1 compiler (built with old rustc, using glibc malloc), doing debug
builds:
```
futures-rs-test  4.684s vs  4.498s --> 1.041x faster (variance: 1.012x, 1.012x)
issue-32062-equ  0.413s vs  0.355s --> 1.162x faster (variance: 1.019x, 1.006x)
issue-32278-big  1.869s vs  1.763s --> 1.060x faster (variance: 1.013x, 1.018x)
jld-day15-parse  1.900s vs  1.602s --> 1.186x faster (variance: 1.010x, 1.003x)
piston-image-0. 12.907s vs 12.352s --> 1.045x faster (variance: 1.005x, 1.006x)
rust-encoding-0  3.254s vs  3.248s --> 1.002x faster (variance: 1.063x, 1.045x)
```
stage2 compiler (built with new rustc, using jemalloc), doing debug builds:
```
futures-rs-test  4.183s vs  4.046s --> 1.034x faster (variance: 1.007x, 1.004x)
issue-32062-equ  0.380s vs  0.340s --> 1.117x faster (variance: 1.020x, 1.003x)
issue-32278-big  1.671s vs  1.616s --> 1.034x faster (variance: 1.031x, 1.012x)
jld-day15-parse  1.661s vs  1.417s --> 1.172x faster (variance: 1.013x, 1.005x)
piston-image-0. 11.347s vs 10.841s --> 1.047x faster (variance: 1.007x, 1.010x)
rust-encoding-0  3.050s vs  3.000s --> 1.017x faster (variance: 1.016x, 1.012x)
```
@eddyb: `git blame` suggests that you should review this. Thanks!

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