[DAGCombiner] Fix to avoid writing outside original store in ReduceLoadOpStoreWidth (#119203) DAGCombiner::ReduceLoadOpStoreWidth could replace memory accesses with more narrow loads/store, although sometimes the new load/store would touch memory outside the original object. That seemed wrong and this patch is simply avoiding doing the DAG combine in such situations. Also simplifying the expression used to align ShAmt down to a multiple of NewBW. Subtracting (ShAmt % NewBW) should do the same thing as the old more complicated expression. Intention is to follow up with a patch that make more attempts, trying to align the memory accesses at other offsets, allowing to trigger the transform in more situations. The current strategy for deciding size (NewBW) and offset (ShAmt) for the narrowed operations are a bit ad-hoc, and not really considering big endian memory order in same way as little endian.
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