[InstCombine] Don't simplify calls without uses

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.

Differential Revision: https://reviews.llvm.org/D75814

lib/Transforms/InstCombine/InstCombineCalls.cpp

@@ -1787,8 +1787,11 @@ Instruction *InstCombiner::foldIntrinsicWithOverflowCommon(IntrinsicInst *II) {
 /// instructions. For normal calls, it allows visitCallBase to do the heavy
 /// lifting.
 Instruction *InstCombiner::visitCallInst(CallInst &CI) {
-  if (Value *V = SimplifyCall(&CI, SQ.getWithInstruction(&CI)))
-    return replaceInstUsesWith(CI, V);
+  // 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);

test/Transforms/InstCombine/pr44245.ll

@@ -59,7 +59,7 @@ define void @test(i1 %c) {
 ; 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