[X86] When post-processing the DAG to remove zero extending moves for YMM/ZMM,...

[X86] When post-processing the DAG to remove zero extending moves for YMM/ZMM, make sure the producing instruction is VEX/XOP/EVEX encoded.

If the producing instruction is legacy encoded it doesn't implicitly zero the upper bits. This is important for the SHA instructions which don't have a VEX encoded version. We might also be able to hit this with the incomplete f128 support that hasn't been ported to VEX.

llvm-svn: 338812