[AArch64] Missed store pairing opportunity in SPEC17 perlbench due to union TBAA

Reduced test case: https://godbolt.org/z/483r6TzEe extracted from SPEC2017 Perlbench.

The stores:
```
rex->lastparen = n;
rex->lastcloseparen = st->u.branch.lastcloseparen;
```
reside adjacent in memory and can be written out with an `stp` but st->u (which is a union access) prevents it from happening.

Similar issue https://github.com/llvm/llvm-project/issues/147449

Start with the reduced test case at https://godbolt.org/z/483r6TzEe and inspect the generated AArch64 code for the two adjacent stores. Investigate how the union access through st->u and TBAA affect store pairing, using issue #147449 for related context. Done means the stores can be emitted as an stp where the alias information permits it, with coverage for the reduced case.

Written by the indexing model from the issue text.