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

[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.

llvm-svn: 365268