[AVX-512] Fix a bad use of a high GR8 register after copying from a mask...

[AVX-512] Fix a bad use of a high GR8 register after copying from a mask register during fast isel. This ends up extracting from bits 15:8 instead of the lower bits of the mask.

I'm pretty sure there are more problems lurking here. But I think this fixes PR32241.

I've added the test case from that bug and added asserts that will fail if we ever try to copy between high registers and mask registers again.

llvm-svn: 297574