[VPlan] Always process all argmin/argmax reductions in plan.
Follow-up to https://github.com/llvm/llvm-project/pull/170223.
Instead of exiting early, continue processing remaining reductions in
the loop. This ensures all multi-use reductions are properly converted
or the plan is rejected if there are unconvertable patterns.
Fixes https://github.com/llvm/llvm-project/issues/184729.
