[AVX-512] Punt on fast-isel of truncates to i1 when AVX512 is enabled.

[AVX-512] Punt on fast-isel of truncates to i1 when AVX512 is enabled.

We should be masking the value and emitting a register copy like we do in non-fast isel. Instead we were just updating the value map and emitting nothing.

After r298928 we started seeing cases where we would create a copy from GR8 to GR32 because the source register in a VK1 to GR32 copy was replaced by the GR8 going into a truncate.

This fixes PR32451.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@298957 91177308-0d34-0410-b5e6-96231b3b80d8