[X86] combineLoad - try to reuse existing constant pool entries for smaller vector constant data

If we already have a YMM/ZMM constant that a smaller XMM/YMM has matching lower bits, then ensure we reuse the same constant pool entry.

Extends the similar combines we already have to reuse VBROADCAST_LOAD/SUBV_BROADCAST_LOAD constant loads.

This is a mainly a canonicalization, but should make it easier for us to merge constant loads in a future commit (related to both #70947 and better X86FixupVectorConstantsPass usage for #71078).