Transforms: Don't create bad weights when eliminating dead cases

If we happen to eliminate every case in a switch that has branch
weights, we currently try to create metadata for the one remaining
branch, triggering an assert. Instead, we need to check that the
metadata we're trying to create is sensible.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197791 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Utils/SimplifyCFG.cpp

@@ -3222,7 +3222,7 @@ static bool EliminateDeadSwitchCases(SwitchInst *SI) {
     Case.getCaseSuccessor()->removePredecessor(SI->getParent());
     SI->removeCase(Case);
   }
-  if (HasWeight) {
+  if (HasWeight && Weights.size() >= 2) {
     SmallVector<uint32_t, 8> MDWeights(Weights.begin(), Weights.end());
     SI->setMetadata(LLVMContext::MD_prof,
                     MDBuilder(SI->getParent()->getContext()).