[clang] [llvm] [HLSL] Implement elementwise firstbitlow builtin (PR #116858)

[Steven Perron via llvm-commits]

https://github.com/s-perron approved this pull request.

Generally looks good to me. Thanks for fixing up the vector size issue. I have a couple minor issue, but I'm fine with this.

https://github.com/llvm/llvm-project/pull/116858