[InstCombine] Don't simplify calls without uses

[nikic]

Summary

When simplifying a call without uses, replaceInstUsesWith() is going to do nothing, but we'll skip all following folds. We can only run into this problem with calls that both simplify and are not trivially dead if unused, which currently seems to happen only with calls to undef, as the test diff shows. When extending SimplifyCall() to handle "returned" attributes, this becomes a much bigger problem, so I'm fixing this first.

[lebedev.ri]

Makes sense to me, but i'm not very familiar with InstCombiner::visitCallInst()

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

if (OptimizeOverflowCheck(WO->getBinaryOp(), WO->isSigned(), WO->getLHS(),
     return CreateOverflowTuple(WO, OperationResult, OverflowResult);
   return nullptr;
 }
 
 /// CallInst simplification. This mostly only handles folding of intrinsic
 /// instructions. For normal calls, it allows visitCallBase to do the heavy
 /// lifting.
 Instruction *InstCombiner::visitCallInst(CallInst &CI) {
+  // Don't try to simplify calls without uses. It will not do anything useful,
+  // but will result in the following folds being skipped.
+  if (!CI.use_empty())
     if (Value *V = SimplifyCall(&CI, SQ.getWithInstruction(&CI)))
       return replaceInstUsesWith(CI, V);
 
   if (isFreeCall(&CI, &TLI))
     return visitFree(CI);
 
   // If the caller function is nounwind, mark the call as nounwind, even if the
   // callee isn't.
   if (CI.getFunction()->doesNotThrow() && !CI.doesNotThrow()) {
     CI.setDoesNotThrow();

llvm/test/Transforms/InstCombine/pr44245.ll

 ; CHECK:       bb147:
 ; CHECK-NEXT:    [[TMP1499:%.*]] = load i8*, i8** inttoptr (i64 16 to i8**), align 16
 ; CHECK-NEXT:    br label [[BB150]]
 ; CHECK:       bb150:
 ; CHECK-NEXT:    [[TMP2]] = phi i8* [ [[TMP1848:%.*]], [[BB152]] ], [ [[TMP1499]], [[BB147]] ]
 ; CHECK-NEXT:    br label [[BB47]]
 ; CHECK:       bb152:
 ; CHECK-NEXT:    [[TMP1848]] = load i8*, i8** inttoptr (i64 16 to i8**), align 16
-; CHECK-NEXT:    call void undef()
+; CHECK-NEXT:    store i1 true, i1* undef, align 1
 ; CHECK-NEXT:    br label [[BB150]]
 ;
 bb16:                                             ; preds = %bb
   br i1 %c, label %bb17, label %bb24
 
 bb17:                                             ; preds = %bb47, %bb17
   %0 = phi i8* [ %1, %bb47 ], [ undef, %bb16 ]
   store i8* %0, i8** undef, align 8