[RISCV] Add addu.w and slliu.w test that uses getelementptr with zero extended indices.

This is closer to the kind of code that these intrinsics are
targeted at. Note we fail to match slliu.w here because our pattern
looks for (and (shl X, C1), 0xffffffff << C1) rather than
(shl (and X, 0xffffffff), C1). I'll fix this in a follow up
commit.