[RISCV] More explicitly check that combineOp_VLToVWOp_VL removes the extends it is supposed to. (#166710)
If we visit multiple root nodes, make sure the strategy chosen
for other nodes includes the nodes we've already committed to remove.
This can occur if have add/sub nodes where one operand is a zext
and the other is a sext. We might see that the nodes share a common
extension but pick a strategy that doesn't share it.