[ControlHeightReduction] Add assert to avoid underflow (llvm#116339)

`NumCHRedBranches - 1` is used later, we should add an assertion to make
sure it will not underflow.

Author: yingcong-wu

llvm/lib/Transforms/Instrumentation/ControlHeightReduction.cpp

@@ -1862,6 +1862,7 @@ void CHR::fixupBranchesAndSelects(CHRScope *Scope,
       ++NumCHRedBranches;
     }
   }
+  assert(NumCHRedBranches > 0);
   Stats.NumBranchesDelta += NumCHRedBranches - 1;
   Stats.WeightedNumBranchesDelta += (NumCHRedBranches - 1) * ProfileCount;
   ORE.emit([&]() {