[ARM][PowerPC] Add regression tests for narrowing load in TargetLowering::SimplifySetCC

These test cases show some miscomplies for big-endian when dealing
with non byte-sized loads. One part of the problem is that LLVM IR
isn't really telling where the padding goes for non byte-sized
loads/stores. So currently TargetLowering::SimplifySetCC can't assume
anything about it. But the implementation also do not consider that
the TypeStoreSize could be larger than the TypeSize, resulting in
the offset calculation being wrong for big-endian.

Pre-commit for https://github.com/llvm/llvm-project/pull/87646