AMDGPU: Merge BUFFER_STORE_DWORD_OFFEN/OFFSET into x2, x4

Summary:
Only 56 shaders (out of 48486) are affected.

Totals from affected shaders (changed stats only):
SGPRS: 2420 -> 2460 (1.65 %)
Spilled VGPRs: 94 -> 112 (19.15 %)
Scratch size: 524 -> 528 (0.76 %) dwords per thread
Code Size: 187400 -> 184992 (-1.28 %) bytes

One DiRT Showdown shader spills 6 more VGPRs.
One Grid Autosport shader spills 12 more VGPRs.

The other 54 shaders only have a decrease in code size.
(I'm ignoring the SGPR noise)

Reviewers: arsenm, nhaehnle

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

Differential Revision: https://reviews.llvm.org/D39012

llvm-svn: 317755