AMDGPU: Unify MOVRELSOffset and MOVRELDOffset

Summary:
Previously, constant index insertelements would be turned into SI_INDIRECT_DST,
which is bound to prevent some optimization opportunities. Worse, it mislead
the heuristic that decides whether immediates should be lowered to S_MOV_B32
or V_MOV_B32 in a way that resulted in unnecessary v_readfirstlanes.

Reviewers: arsenm, tstellarAMD

Subscribers: arsenm, kzhuravl, llvm-commits

Differential Revision: http://reviews.llvm.org/D22217

llvm-svn: 275160
5 files changed