AMDGPU/SI: Set INDEX_STRIDE for scratch coalescing

Summary:
Mesa and other users must set this to enable coalescing:
- STRIDE = 0
- SWIZZLE_ENABLE = 1

This makes one particular compute shader 8x faster.

Reviewers: tstellarAMD, arsenm

Subscribers: arsenm, kzhuravl

Differential Revision: http://reviews.llvm.org/D21136

llvm-svn: 272556