[RISCV][NFC] Add test case for SLP reduction vectorization failure

Horizontal reductions still occur on RISC-V, despite the maximum SLP VF
reported back by TTI being 1, to disable SLP.
This can cause the cost model to think it can vectorize a gather into
smaller, widened loads, when it will actually fail to do so.
This should ultimately be fixed whenever SLP is re-enabled for RISC-V at
some point.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D146529