[AVX-512] Add lowering to cvttpd2udq/cvttps2udq for fptoui v2f64/2f32 to 2i32

This patch adds support for fptoui to 2i32 from both 2f64 and 2f32, building on Simon's change for the signed version in r284459 and using AVX-512 instructions.

If we don't have VLX support we need to use a 512-bit operation for v2f64->v2i32 and extract the result.

It also recognises that cvttpd2udq zeroes the upper 64-bits of the xmm result.

Differential Revision: https://reviews.llvm.org/D26331

llvm-svn: 286345