[llvm] [VPlan] Don't use the legacy cost model for loop conditions (PR #156864)

[John Brawn via llvm-commits]

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@@ -9,11 +9,12 @@ define i64 @test(ptr %a, ptr %b) #0 {
 ; CHECK-LABEL: LV: Checking a loop in 'test'
 ; CHECK: Cost of 1 for VF 8: induction instruction   %i.iv.next = add nuw nsw i64 %i.iv, 1
 ; CHECK-NEXT: Cost of 0 for VF 8: induction instruction   %i.iv = phi i64 [ 0, %entry ], [ %i.iv.next, %for.body ]
-; CHECK-NEXT: Cost of 1 for VF 8: exit condition instruction   %exitcond.not = icmp eq i64 %i.iv.next, 16
 ; CHECK-NEXT: Cost of 0 for VF 8: EMIT vp<{{.+}}> = CANONICAL-INDUCTION ir<0>, vp<%index.next>
+; CHECK: Cost of 1 for VF 8: EMIT branch-on-count vp<%index.next>, vp<{{.+}}>
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john-brawn-arm wrote:

It looks like that function is marking both compares and induction variable instructions as needing to be ignored. So we could remove the ignoring of compares, except that the function is also called from the legacy cost model which does need to continue ignoring the compares. So I think the call to addFullyUnrolledInstructionsToIgnore in LoopVectorizationPlanner::precomputeCosts can only be removed once the vplan cost model is also correctly handling the induction variable instructions.

https://github.com/llvm/llvm-project/pull/156864