[RISCV][NFC] Make interleaved access test more vectorizable

The previous test case stored the result of a deinterleaved load and add
into the same source address, which resulted in some scatters which we
weren't testing for and made the tests harder to understand.
Store it at a separate address, which will make the tests easier to read
when the cost model is changed after D145085 is landed

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D146442