AMDGPU: Merge BUFFER_LOAD_DWORD_OFFEN into x2, x4

Summary:
-9.9% code size decrease in affected shaders.

Totals (changed stats only):
SGPRS: 2151462 -> 2170646 (0.89 %)
VGPRS: 1634612 -> 1640288 (0.35 %)
Spilled SGPRs: 8942 -> 8940 (-0.02 %)
Code Size: 52940672 -> 51727288 (-2.29 %) bytes
Max Waves: 373066 -> 371718 (-0.36 %)

Totals from affected shaders:
SGPRS: 283520 -> 302704 (6.77 %)
VGPRS: 227632 -> 233308 (2.49 %)
Spilled SGPRs: 3966 -> 3964 (-0.05 %)
Code Size: 12203080 -> 10989696 (-9.94 %) bytes
Max Waves: 44070 -> 42722 (-3.06 %)

Reviewers: arsenm, nhaehnle

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

Differential Revision: https://reviews.llvm.org/D38950

llvm-svn: 317752