AMDGPU: Fold immediate offset into BUFFER_LOAD_DWORD lowered from SMEM

Summary:
-5.3% code size in affected shaders.

Changed stats only:

48486 shaders in 30489 tests
Totals:
SGPRS: 2086406 -> 2072430 (-0.67 %)
VGPRS: 1626872 -> 1627960 (0.07 %)
Spilled SGPRs: 7865 -> 7912 (0.60 %)
Code Size: 60978060 -> 60188764 (-1.29 %) bytes
Max Waves: 374530 -> 374342 (-0.05 %)

Totals from affected shaders:
SGPRS: 299664 -> 285688 (-4.66 %)
VGPRS: 233844 -> 234932 (0.47 %)
Spilled SGPRs: 3959 -> 4006 (1.19 %)
Code Size: 14905272 -> 14115976 (-5.30 %) bytes
Max Waves: 46202 -> 46014 (-0.41 %)

Reviewers: arsenm, nhaehnle

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

Differential Revision: https://reviews.llvm.org/D38915

llvm-svn: 317750