[SLP] Lit test that cannot get vectorized due to lack of look-ahead operand reordering heuristic.

The code in this test is not vectorized by SLP because its operand reordering cannot look beyond the immediate predecessors.
This will get fixed in a follow-up patch that introduces the look-ahead operand reordering heuristic.

Committed on behalf of @vporpo (Vasileios Porpodas)

Differential Revision: https://reviews.llvm.org/D61283

llvm-svn: 359553