AMDGPU: Split MUBUF offset into aligned components

Summary:
Atomic buffer operations do not work (and trap on gfx9) when the
components are unaligned, even if their sum is aligned.

Previously, we generated an offset of 4156 without an SGPR by
splitting it as 4095 + 61 (immediate + inline constant). The
highest offset for which we can do this correctly is 4156 = 4092 + 64.

Fixes dEQP-GLES31.functional.ssbo.atomic.*

Reviewers: arsenm

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

Differential Revision: https://reviews.llvm.org/D37850

llvm-svn: 315302