Complete support for containment of memory operands under casts #74490

#72719 added basic support for containment of memory operands under casts. We can and should extend it.

1. Contain indirections with complex addressing modes on ARM/ARM64. The complicating factor is that the rules for when a certain addressing mode can be contained are quite complex on these architectures, and cast containment can effectively alter the load width. We should factor the code such that deciding when the containment is still possible and profitable is easy.
2. Enable containment for LA64 and abstract the legality check in lowering.

category:implementation
theme:memory-usage