[X86] Use X86ISD::VFPROUND instead of ISD::FP_ROUND for 256 and 512 bit cvtpd2ps intrinsics.

Summary:
Use X86ISD::VFPROUND in the instruction isel patterns. Add new patterns for ISD::FP_ROUND to maintain support for fptrunc in IR.

In the process I found a couple duplicate isel patterns which I also deleted in this patch.

Reviewers: RKSimon, spatel

Reviewed By: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D56991

llvm-svn: 351762