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

[Steven Perron via cfe-commits]

https://github.com/s-perron commented:

There is only potential problem with the vector sizes. Sorry I did not catch that on the previous PR. I would also like to see the code refactored a little to reduce the amount of repeated code. I think each step except for 2 in `selectFirstBit*64` could factored into a function call. The difference between the High and Low is a small change in the parameters to those function calls.

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