Tweak `SlicePartialEq` to allow MIR-inlining the `compare_bytes` call rust-lang/rust#150265 disabled this because it was a net perf win, but let's see if we can tweak the structure of this to allow more inlining on this side while still not MIR-inlining the loop when it's not just `memcmp` and thus hopefully preserving the perf win. This should also allow MIR-inlining the length check, which was previously blocked, and thus might allow some obvious non-matches to optimize away as well.