Skip deadblocks in cycle saving computation

llvm/lib/Analysis/InlineCost.cpp

std::optional<bool> costBenefitAnalysis() {
     // We use 128-bit APInt here to avoid potential overflow.  This variable
     // should stay well below 10^^24 (or 2^^80) in practice.  This "worst" case
     // assumes that we can avoid or fold a billion instructions, each with a
     // profile count of 10^^15 -- roughly the number of cycles for a 24-hour
     // period on a 4GHz machine.
     APInt CycleSavings(128, 0);
 
     for (auto &BB : F) {
+      // Skip dead blocks since they are going to be simplified away.
+      if (DeadBlocks.count(&BB))
+       continue;
       APInt CurrentSavings(128, 0);
       for (auto &I : BB) {
         if (BranchInst *BI = dyn_cast<BranchInst>(&I)) {
           // Count a conditional branch as savings if it becomes unconditional.
           if (BI->isConditional() &&
               isa_and_nonnull<ConstantInt>(
                   SimplifiedValues.lookup(BI->getCondition()))) {
             CurrentSavings += InstrCost;