[X86] Remove GCCBuiltins from 512-bit cvt(u)qqtops, cvt(u)qqtopd, and...

[X86] Remove GCCBuiltins from 512-bit cvt(u)qqtops, cvt(u)qqtopd, and cvt(u)dqtops intrinsics. Add new variadic uitofp/sitofp with rounding mode intrinsics.

Summary: See clang patch D56998 for a full description.

Reviewers: RKSimon, spatel

Reviewed By: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D56999

llvm-svn: 352266