[CodeGen] Don't crash on for loops with cond variables and no increment

[bkramer]

Summary

This looks like an oversight from a875721d8a2d, creating IR that refers
to for.inc even if it doesn't exist.

[rupprecht]

Thanks!

[bkramer]

Landing this as it's a pretty bad crasher. Feel free to point out all my mistakes in post-commit review :)

[rjmccall]

Yes, LGTM.

clang/lib/CodeGen/CGStmt.cpp

void CodeGenFunction::EmitForStmt(const ForStmt &S,
   if (S.getCond()) {
     // If the for statement has a condition scope, emit the local variable
     // declaration.
     if (S.getConditionVariable()) {
       EmitDecl(*S.getConditionVariable());
 
       // We have entered the condition variable's scope, so we're now able to
       // jump to the continue block.
-      Continue = getJumpDestInCurrentScope("for.inc");
+      Continue = S.getInc() ? getJumpDestInCurrentScope("for.inc") : CondDest;
       BreakContinueStack.back().ContinueBlock = Continue;
     }
 
     llvm::BasicBlock *ExitBlock = LoopExit.getBlock();
     // If there are any cleanups between here and the loop-exit scope,
     // create a block to stage a loop exit along.
     if (ForScope.requiresCleanups())
       ExitBlock = createBasicBlock("for.cond.cleanup");

clang/test/CodeGenCXX/for-cond-var.cpp

void PR49585_break() {
   // CHECK-NEXT: label %[[for_end:.*]]
 
   // CHECK: [[cleanup_cont]]:
   // CHECK: br label %[[for_cond]]
 
   // CHECK [[for_end]]:
   // CHECK: ret void
 }
+
+// CHECK: define {{.*}} void @_Z16incless_for_loopv(
+void incless_for_loop() {
+  // CHECK: br label %[[for_cond:.*]]
+  // CHECK: [[for_cond]]:
+  // CHECK:   br i1 {{.*}}, label %[[for_body:.*]], label %[[for_end:.*]]
+  // CHECK: [[for_body]]:
+  // CHECK:   br label %[[for_cond]]
+  // CHECK: [[for_end]]:
+  // CHECK:   ret void
+  // CHECK: }
+  for (; int b = 0;) continue;
+}