[X86] Use __builtin_convertvector to implement some of the packed integer to...

[X86] Use __builtin_convertvector to implement some of the packed integer to packed float conversion intrinsics.

I believe this is safe assuming default default FP environment. The conversion might be inexact, but it can never overflow the FP type so this shouldn't be undefined behavior for the uitofp/sitofp instructions.

We already do something similar for scalar conversions.

Differential Revision: https://reviews.llvm.org/D46863

llvm-svn: 332882