AMDGPU/R600: Remove code for handling AMDGPUISD::CLAMP

Summary:
We don't generate AMDGPUISD::CLAMP for R600 now that llvm.AMDGPU.clamp
is gone.

Reviewers: arsenm, nhaehnle

Reviewed By: arsenm

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

Differential Revision: https://reviews.llvm.org/D47181

llvm-svn: 333153