[AMDGPU] add intrinsic for s_getpc

Summary: The s_getpc instruction is exposed as intrinsic llvm.amdgcn.s.getpc.

Reviewers: arsenm

Reviewed By: arsenm

Subscribers: kzhuravl, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye

Differential Revision: https://reviews.llvm.org/D32862

llvm-svn: 303859