[X86] Add CMOV_VR64 pseudo instruction for MMX. Remove mmx handling from combineSelect.

The combineSelect code was casting to i64 without any check that
i64 was legal. This can break after type legalization.

It also required splitting the mmx register on 32-bit targets.
It's not clear that this makes sense. Instead switch to using
a cmov pseudo like we do for XMM/YMM/ZMM.