[RISCV] Add test cases for folding disjoint Or into a scalar load address. NFC

After 47a1704a we are able to
reassociate a disjoint Or used as a GEP index to get the constant
closer to a load to fold it. This is show by the first test.

We are not able to do this if the GEP created a shift left to scale
the index as the second test shows.

To make this work, we need to preserve the disjoint flag when pulling
the Or through the shift.