[MMX] Use the appropriate instructions for GR64 <-> VR64 copies.

MOVSDto64rr and MOV64toSDrr are defined to convert between FR64 (%xmm)
<-> GR64 registers, not VR64 (%mm) <-> GR64. This is wrong.

I found this by inspection and could not find a suitable testcase for it
since (1) we don't handle MMX bitcasts in Peephole optimizer as to
generate COPYs that (2) could be expanded back to the appropriate x86
instruction in ExpandPostRA.

Switch to use the appropriate instructions: MMX_MOVD64from64rr and
MMX_MOVD64to64rr here.

llvm-svn: 242191