[DAG] LoadedSlice::canMergeExpensiveCrossRegisterBankCopy - replace...

[DAG] LoadedSlice::canMergeExpensiveCrossRegisterBankCopy - replace getABITypeAlign with allowsMemoryAccess (PR45116)

One of the cases identified in PR45116 - we don't need to limit load combines to ABI alignment, we can use allowsMemoryAccess - which tests using getABITypeAlign, but also checks if a target permits (fast) misaligned memory loads by checking allowsMisalignedMemoryAccesses as a fallback.