[TypePromotion] Don't mutate the result type of SwitchInst.

[craig.topper]

Summary

SwitchInst should have a void result type.

Add a check to the verifier to catch this error.

[samparker]

Thanks.

llvm/lib/CodeGen/TypePromotion.cpp

for (unsigned i = 0, e = I->getNumOperands(); i < e; ++i) {
 
       if (auto *Const = dyn_cast<ConstantInt>(Op)) {
         Constant *NewConst = ConstantExpr::getZExt(Const, ExtTy);
         I->setOperand(i, NewConst);
       } else if (isa<UndefValue>(Op))
         I->setOperand(i, UndefValue::get(ExtTy));
     }
 
-    // Mutate the result type, unless this is an icmp.
-    if (!isa<ICmpInst>(I)) {
+    // Mutate the result type, unless this is an icmp or switch.
+    if (!isa<ICmpInst>(I) && !isa<SwitchInst>(I)) {
       I->mutateType(ExtTy);
       Promoted.insert(I);
     }
   }
 }
 
 void IRPromoter::TruncateSinks() {
   LLVM_DEBUG(dbgs() << "IR Promotion: Fixing up the sinks:\n");

llvm/lib/IR/Verifier.cpp

Assert(N == 1 && F->getReturnType() == RI.getOperand(0)->getType(),
            &RI, F->getReturnType());
 
   // Check to make sure that the return value has necessary properties for
   // terminators...
   visitTerminator(RI);
 }
 
 void Verifier::visitSwitchInst(SwitchInst &SI) {
+  Assert(SI.getType()->isVoidTy(), "Switch must have void result type!", &SI);
   // Check to make sure that all of the constants in the switch instruction
   // have the same type as the switched-on value.
   Type *SwitchTy = SI.getCondition()->getType();
   SmallPtrSet<ConstantInt*, 32> Constants;
   for (auto &Case : SI.cases()) {
     Assert(Case.getCaseValue()->getType() == SwitchTy,
            "Switch constants must all be same type as switch value!", &SI);
     Assert(Constants.insert(Case.getCaseValue()).second,