[X86] Change the MOV32ri64 pseudo instruction to def a GR64 directly instead...

[X86] Change the MOV32ri64 pseudo instruction to def a GR64 directly instead of wrapping it in a SUBREG_TO_REG.

Now we switch to the subregister in expandPostRAPseudos where we already switched the opcode.

This simplifies a few isel patterns that used the pseudo directly. And magically seems to have improved our ability to CSE it in the undef-label.ll test.

llvm-svn: 339496