[DAGCombine] Refactor DAGCombiner::ReduceLoadWidth. NFCI

Update code comments in DAGCombiner::ReduceLoadWidth and refactor
the handling of SRL a bit. The refactoring is done with the intent
of adding support for folding away SRA by using SEXTLOAD in a
follow-up patch.

The function is also renamed as DAGCombiner::reduceLoadWidth.

Differential Revision: https://reviews.llvm.org/D117104