[X86] Use __builtin_convertvector to implement _mm512_cvtps_pd.

If we're using default rounding mode we can let __builtin_convertvector to generate an fpextend. This matches 128 and 256 bit.

If we're using the version that takes an explicit rounding mode argument we would need to look at the immediate to see if its CUR_DIRECTION.

llvm-svn: 332210