[OpaquePtr][ARM] Use elementtype on ldrex/ldaex/stlex/strex

Includes verifier changes checking the elementtype, clang codegen
changes to emit the elementtype, and ISel changes using the elementtype.

Basically the same as D120527.

Reviewed By: #opaque-pointers, nikic

Differential Revision: https://reviews.llvm.org/D121847