[DAGCombiner] More load-store forwarding for big-endian

[DAGCombiner] More load-store forwarding for big-endian

Get some load-store forwarding cases for big-endian where a larger store covers
a smaller load, and the offset would be 0 and handled on little-endian but on
big-endian the offset is adjusted to be non-zero. The idea is just to shift the
data to make it look like the offset 0 case.

Differential Revision: https://reviews.llvm.org/D130115