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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183971 91177308-0d34-0410-b5e6-96231b3b80d8