[llvm] [X86] Use BMM for bitreverse without GFNI (PR #209223)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

> > thanks! sorry for the back and forth :D
> 
> Thanks for opening this PR. There is CTLZ(x) = CTTZ(BITREVERSE(x)) gap with bmm. Also, scalar bitreverse i8 is not using bitrevb when we have bmm. I will tackle these separately.

Hopefully we  can handle most of this generically in DAGCombine

https://github.com/llvm/llvm-project/pull/209223