[X86] Add PS<->PD domain changing support for MOVH/MOVL load instructions and MOVH...

[X86] Add PS<->PD domain changing support for MOVH/MOVL load instructions and MOVH store instructions.

These instructions don't have an integer domain equivalent, but
we can at least change between the two floating point domains.

This allows a smaller encoding on SSE targets if we can turn
PD into PS.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@365268 91177308-0d34-0410-b5e6-96231b3b80d8