[X86] Fold MMX_MOVD64from64rr + store to MMX_MOVQ64mr instead of MMX_MOVD64from64mr.

MMX_MOVD64from64rr moves an MMX register to a 64-bit GPR.

MMX_MOVD64from64mr is the memory version of moving a MMX register to a
64-bit GPR. It requires the REX.W bit to be set. There are no isel
patterns that use this instruction.

MMX_MOVQ64mr is the MMX register store instruction. It doesn't
require a REX.W prefix. This makes it one byte shorter to encode
than MMX_MOVD64from64mr in many cases.

Both store instructions output the same mnemonic string. The assembler
would choose MMX_MOVQ64mr if it was to parse the output. Which is
another reason using it is the correct thing to do.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D122241