ARM: handle post-indexed NEON ops where the offset isn't the access width.

ARM: handle post-indexed NEON ops where the offset isn't the access width.

Before, we assumed that any ConstantInt offset was precisely the access width,
so we could use the "[rN]!" form. ISelLowering only ever created that kind, but
further simplification during combining could lead to unexpected constants and
incorrect codegen.

Should fix PR32658.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@300878 91177308-0d34-0410-b5e6-96231b3b80d8