[X86][MMX] Remove completed _mm_cvtsi32_si64 todo

rL322525 - mmx zero constant support
rL322553 - mmx i32 zero extended value
rL326497 - mmx i64 general constant handling

Not all constants are folded, we generate some on the GPRs (similar to SSE build vector) where appropriate

llvm-svn: 326673