[llvm] [InlineCost] Consider the default branch when calculating cost (PR #77856)

[Quentin Dian via llvm-commits]

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@@ -3505,6 +3506,10 @@ class SwitchInst : public Instruction {
     return cast<BasicBlock>(getOperand(1));
   }
 
+  bool defaultDestIsUnreachable() const {
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DianQK wrote:

Thanks for the reminder. I prefer `defaultDestUndefined`.

https://github.com/llvm/llvm-project/pull/77856