X86: cvtpi2ps is just an SSE instruction with MMX operands. It has no AVX
equivalent.
Give it the right register format so we can also emit it when AVX is enabled.
This should fix a "Cannot select: intrinsic %llvm.x86.sse.cvtpi2ps" fatal error
in clang while building the gnuradio port for amd64.
Reported by: db
MFC after: 3 days
b836cde101