AMDGPU: Move AMDGPUTargetLowering::isFPExtFoldable() into SITargetLowering

Summary: This is always false for R600.

Reviewers: arsenm, nhaehnle

Reviewed By: arsenm

Subscribers: kzhuravl, wdng, yaxunl, dstuttard, tpr, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D47180

llvm-svn: 333016