R600/SI: Consider adjacent offsets in getLdStBaseRegImmOfs

We can treat ds_read2_* as a single offset if the offsets are adjacent.

No test since emission of read2 instructions for partially
aligned loads isn't implemented yet.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214269 91177308-0d34-0410-b5e6-96231b3b80d8