[llvm] [RFC][X86] Allow speculative BSR/BSF instructions on targets with CMOV (PR #102885)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

Interesting, hopefully we can make use of that properly, even if we just support it with an opt-in feature flag.

I noticed that GCC doesn't use the ZF result from the BSF/BSR instruction and instead performs the zero-input calculation separately: https://rust.godbolt.org/z/oTf4fqWvs

https://github.com/llvm/llvm-project/pull/102885