[X86] combineCMP - attempt to simplify KSHIFTR mask element extractions when...

[X86] combineCMP - attempt to simplify KSHIFTR mask element extractions when just comparing against zero

We can just bitcast the pre-shifted mask as an integer and use TEST/BT directly.

This can be extended further to better handle sub-i8 mask cases, but just getting rid of KSHIFTR nodes makes a notable difference.