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

[Quentin Dian via llvm-commits]

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

https://github.com/llvm/llvm-project/commit/5932fcc47855fdd209784f38820422d2369b84b2#r138510755
cc @nikic 

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