[X86] combineLoad - reuse an existing VBROADCAST_LOAD constant for a smaller...

[X86] combineLoad - reuse an existing VBROADCAST_LOAD constant for a smaller vector load of the same constant

Extends the existing code that performed something similar for SUBV_BROADCAST_LOAD, but this is just for cases where AVX2 targets loads full width 128-bit constant vectors but broadcasts the equivalent 256-bit constant vector

Fixes AVX2 case for Issue #70947