AMDGPU/SI: Define S_GETREG Intrinsic

Summary:
 Define s_getreg intrinsic to generate s_getreg instruction to read
hardware registers.

Reviewers: tstellarAMD, arsenm

Subscribers: llvm-commits, arsenm

Differential Revision: http://reviews.llvm.org/D17892

llvm-svn: 263124