[RISCV] Add test cases showing inefficient materialization for stores of immediates. NFC

DAGCombiner::visitStore can call GetDemandedBits which will remove
upper bits from immediates. The upper bits are important for good
materialization of negative constants on RISCV. GetDemandedBits is a
different mechanism than SimplifyDemandedBits so
TargetShrinkDemandedConstant can't block it.

As far as I know this behavior is unique to stores.

I think we can fix this in isel using a concept similar to D107658.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D107860