[RISCV] Add tests for combineBinOpOfZExts. NFC (#86689)

Unlike add, sub and mul, we don't have widening instructions for div,
rem and logical ops, so we don't have any test coverage if we were to
extend combineBinOpOfZExts to handle them.

Adding tests coincidentally revealed that logical ops are already
narrowed as a generic DAG combine via
DAGCombiner::hoistLogicOpWithSameOpcodeHands. So we don't actually need
to run combineBinOpOfZExts on them.