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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

> sorry @RKSimon i saw that the block was guard by `hasBWI` which is also not need for this to work, i.e. `; RUN: llc < %s -mtriple=x86_64-unknown -mattr=+avx512vl,+avx512bmm | FileCheck %s --check-prefixes=CHECK,BMM` should also use bitrev.. so it has to be a new block

BMM is dependent on BWI (for the mask registers) so keeping it in the original BMM block is fine - thats why the vXi8 legal cases are already there

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