[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.

llvm-svn: 298957