[SelectionDAG] Unify scalarizeVectorLoad and VectorLegalizer::ExpandLoad

The two code paths have the same goal, legalizing a load of a non-byte-sized vector by loading the "flattened" representation in memory, slicing off each single element and then building a vector out of those pieces.

The technique employed by `ExpandLoad`  is slightly more convoluted and produces slightly better codegen on ARM, AMDGPU and x86 but suffers from some bugs (D78480) and is wrong for BE machines.

Differential Revision: https://reviews.llvm.org/D79096