[X86] Add DAG combine to turn (v2i64 (scalar_to_vector (i64 (bitconvert...

[X86] Add DAG combine to turn (v2i64 (scalar_to_vector (i64 (bitconvert (mmx))))) to MOVQ2DQ. Remove unneeded isel patterns.

We already had a DAG combine for (mmx (bitconvert (i64 (extractelement v2i64))))
to MOVDQ2Q.

Remove patterns for MMX_MOVQ2DQrr/MMX_MOVDQ2Qrr that use
scalar_to_vector/extractelement involving i64 scalar type with
v2i64 and x86mmx.