[VPlan] Only use legacy cost for instructions only used by exit conds. (#174029) Currently we need to precompute costs for exit conditions, to match the legacy cost, as they will get replaced by a compare against the canonical IV (or others, like active-lane-mask or EVL based) and the original compare will get removed. This is not true for instructions with users other than the exit condition. Those will remain, and we can just use the VPlan-based cost model to get more accurate results. This improves results in some cases, like @test_value_in_exit_compare_chain_used_outside because the IV increment user outside the loop is replaced by computing the final value outside the loop. It also fixes a crash introduced by f196b1d66ff (#146525). PR: https://github.com/llvm/llvm-project/pull/174029
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