AMDGPU: Fix splitting vector loads with existing offsets

If the original MMO had an offset, it was dropped.
Also use the correct alignment after adding the new offset.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@255508 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed