[llvm] [IR][NFC] Use `SwitchInst::defaultDestUnreachable` (PR #134199)

[via llvm-commits]

================
@@ -6237,11 +6235,8 @@ static bool initializeUniqueCases(SwitchInst *SI, PHINode *&PHI,
   // is unreachable.
   DefaultResult =
       DefaultResults.size() == 1 ? DefaultResults.begin()->second : nullptr;
-  if ((!DefaultResult &&
-       !isa<UnreachableInst>(DefaultDest->getFirstNonPHIOrDbg())))
----------------
dianqk wrote:

error: unused variable 'DefaultDest'

https://github.com/llvm/llvm-project/pull/134199