[AMDGPU] Extend SILoadStoreOptimizer to handle global loads

There can be situations where global and flat loads and stores are not
combined by the vectorizer, in particular if their address space
differ in the IR but they end up the same class instructions after
selection. For example a divergent load from constant address space
ends up being the same global_load as a load from global address space.

TODO: merge global stores.
TODO: handle SADDR forms.
TODO: merge flat load/stores.
TODO: merge flat with global promoting to flat.

Differential Revision: https://reviews.llvm.org/D120279