[DAG] CombineConsecutiveLoads - replace getABITypeAlign with allowsMemoryAccess (PR45116)

One of the cases identified in PR45116 - we don't need to limit load combines (in this case for ISD::BUILD_PAIR) 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.

This helps in particular for 32-bit X86 cases loading 64-bit size data, reducing codegen diffs vs x86_64.

Differential Revision: https://reviews.llvm.org/D108307