[AMDGPU] SILoadStoreOptimizer: rewrite checkAndPrepareMerge. NFCI.

Separate the function clearly into:
- Checks that can be done on CI and Paired before the loop.
- The loop over all instructions between CI and Paired.
- Checks that must be done on InstsToMove after the loop.

Previously these were mostly done inside the loop in a very
confusing way.

Differential Revision: https://reviews.llvm.org/D118994