[llvm] [LoopInterchange] Ignore the cost-model, force interchange if legal (PR #148858)

[Ryotaro Kasuga via llvm-commits]

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@@ -1311,6 +1324,9 @@ bool LoopInterchangeProfitability::isProfitable(
       shouldInterchange =
           isProfitableForVectorization(InnerLoopId, OuterLoopId, DepMatrix);
       break;
+    case RuleTy::Ignore:
+      // Nothing to do, this has no effect.
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kasuga-fj wrote:

Maybe using `llvm_unreachable` is better?

https://github.com/llvm/llvm-project/pull/148858