GlobalISel: IRTranslate llvm.fabs.* intrinsic

Summary:
Fabs is a common floating-point operation, especially for some expansions. This patch adds
a new generic opcode for llvm.fabs.* intrinsic in order to avoid building/matching this intrinsic.

Reviewers: qcolombet, aditya_nandakumar, dsanders, rovka

Reviewed By: aditya_nandakumar

Subscribers: kristof.beyls, javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D43864

llvm-svn: 326749