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

[John Brawn via llvm-commits]

================
@@ -6988,6 +6948,35 @@ static bool planContainsAdditionalSimplifications(VPlan &Plan,
     });
   });
 }
+
+static bool planContainsDifferentCompares(VPlan &Plan, VPCostContext &CostCtx,
+                                          Loop *TheLoop, ElementCount VF) {
+  // Count how many compare instructions there are in the legacy cost model.
+  unsigned NumLegacyCompares = 0;
+  for (BasicBlock *BB : TheLoop->blocks()) {
+    for (auto &I : *BB) {
+      if (isa<CmpInst>(I)) {
+        NumLegacyCompares += 1;
+      }
+    }
+  }
+
+  // Count how many compare instructions there are in the VPlan.
+  unsigned NumVPlanCompares = 0;
+  auto Iter = vp_depth_first_deep(Plan.getVectorLoopRegion()->getEntry());
+  for (VPBasicBlock *VPBB : VPBlockUtils::blocksOnly<VPBasicBlock>(Iter)) {
----------------
john-brawn-arm wrote:

I've changed this to ignore blocks outside of the vector loop region.

> I think it will also always disable the check if we have loops controlled by active-lane-mask?

I'm not sure what you're asking here. When we have a loop that's using an active-lane-mask, the vplan will have something like (example here taken from llvm/test/Transforms/LoopVectorize/AArch64/sve-wide-lane-mask.ll)
```
Cost of 1 for VF vscale x 4: EMIT vp<%active.lane.mask.next> = active lane mask vp<%10>, vp<%4>, ir<1>
Cost of 0 for VF vscale x 4: EMIT vp<%11> = not vp<%active.lane.mask.next>
Cost of 0 for VF vscale x 4: EMIT branch-on-cond vp<%11>
```
the legacy cost model will have
```
LV: Found an estimated cost of 1 for VF vscale x 4 For instruction:   %exitcond.not = icmp eq i64 %iv.next, %n
LV: Found an estimated cost of 0 for VF vscale x 4 For instruction:   br i1 %exitcond.not, label %for.end, label %for.body
```
planContainsDifferentCompares would count 1 compare in the legacy cost model, no compares in the vplan, and return true.

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