[AMDGPU/GlobalISel] Add llvm.amdgcn.fdiv.fast legalization.

Reviewers: arsenm

Reviewed By: arsenm

Subscribers: volkan, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, rovka, dstuttard, tpr, t-tye, hiraditya, Petar.Avramovic, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64966

llvm-svn: 367344