[SDAG] clean up scalarizing load transform

I have not found a way to expose a difference for this patch in a test
because it only triggers for a one-use load, but this is the code that
was adapted into D118376 and caused miscompiles. The new code pattern
is the same as what we do in narrowExtractedVectorLoad() (reduces load
width for a subvector extract).

This removes seemingly unnecessary manual worklist management and fixes
the chain updating via "SelectionDAG::makeEquivalentMemoryOrdering()".

Differential Revision: https://reviews.llvm.org/D119549