rollup merge of #24846: dotdash/fast_cttz8

rollup merge of #24846: dotdash/fast_cttz8

Currently, LLVM lowers a cttz8 on x86_64 to these instructions:

```asm
    movzbl      %dil, %eax
    bsfl        %eax, %eax
    movl        $32, %ecx
    cmovnel     %eax, %ecx
    cmpl        $32, %ecx
    movl        $8, %eax
    cmovnel     %ecx, %eax
```

To improve the codegen, we can zero extend the 8 bit integer, then set
bit 8 and perform a cttz operation on the extended value. That way
there's no conditional operation involved at all.

This was discovered by  this benchmark: https://github.com/Kimundi/long_strings_without_repeats

Timings on my box with the current nightly:
```
running 4 tests
test bench_cpp_naive_big     ... bench:   5479222 ns/iter (+/- 254222)
test bench_noop_big          ... bench:    571405 ns/iter (+/- 111950)
test bench_rust_naive_big    ... bench:   7798102 ns/iter (+/- 148841)
test bench_rust_unsafe_big   ... bench:   6606488 ns/iter (+/- 67529)
```

Timings with the patch applied:
```
running 4 tests
test bench_cpp_naive_big     ... bench:   5470944 ns/iter (+/- 7109)
test bench_noop_big          ... bench:    568944 ns/iter (+/- 6895)
test bench_rust_naive_big    ... bench:   6795901 ns/iter (+/- 43806)
test bench_rust_unsafe_big   ... bench:   5584879 ns/iter (+/- 5291)
```