[SPARC] Fix fp128 load/stores

The generated code for the split fp128 load/stores was missing a small yet important adjustment to the pointer metadata being fed into `getStore` and `getLoad`, making it out of sync with the effective memory address.
This problem often resulted in instructions being scheduled in the wrong order.

I also took this chance to clean up some "wrong" uses of `getAlignment` as done in D77687.

Thanks @jrtc27 for finding the problem and providing a patch.

Patch by LemonBoy and Jessica Clarke(jrtc27)

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D94345