[X86][MMX] Prevent MMX_MOVD64rm folding

MMX_MOVD64rm zero-extends i32 load results into i64 registers.

The peephole optimizer will try to fold it in other MMX foldable
instructions, the wrong thing to do, since there's no MMX memory
instruction that loads from i32 and does implict zero extension.

Remove 'canFoldAsLoad' from MOVD64rm in order to prevent such folding.
The current MMX tests already test this, but since there are no MMX
instructions in the foldable tables yet, this did not trigger. This
commit prepares the addition of those instructions.

llvm-svn: 230498