[X86] Remove isel pattern for MMX_X86movdq2q+simple_load. Replace with DAG combine to to loadmmx.
Only 64-bit bits will be loaded, not the whole 128 bits. We can
just combine it to plain mmx load. This has the side effect of
enabling isel load folding for it.
This part of my desire to get rid of isel patterns that shrink loads.