[DebugInfo][InstMerge] Fix -debugify for phi node created by -mldst-motion

[rupprecht]

Summary

-mldst-motion creates a new phi node without any debug info. Use the merged debug location from the incoming stores to fix this.

Fixes PR38177. The test case here is (somewhat) simplified from:

struct S {
  int foo;
  void fn(int bar);
};
void S::fn(int bar) {
  if (bar)
    foo = 1;
  else
    foo = 0;
}

[vsk]

Thanks, lgtm!

lib/Transforms/Scalar/MergedLoadStoreMotion.cpp

PHINode *MergedLoadStoreMotion::getPHIOperand(BasicBlock *BB, StoreInst *S0,
   // Create a phi if the values mismatch.
   Value *Opd1 = S0->getValueOperand();
   Value *Opd2 = S1->getValueOperand();
   if (Opd1 == Opd2)
     return nullptr;
 
   auto *NewPN = PHINode::Create(Opd1->getType(), 2, Opd2->getName() + ".sink",
                                 &BB->front());
+  NewPN->applyMergedLocation(S0->getDebugLoc(), S1->getDebugLoc());
   NewPN->addIncoming(Opd1, S0->getParent());
   NewPN->addIncoming(Opd2, S1->getParent());
   return NewPN;
 }
 
 ///
 /// Merge two stores to same address and sink into \p BB
 ///

test/Transforms/InstMerge/st_sink_check_debug.ll

+; RUN: opt < %s -S -debugify -mldst-motion -o - | FileCheck %s
+
+%struct.S = type { i32 }
+
+define dso_local void @foo(%struct.S* %this, i32 %bar) {
+entry:
+  %this.addr = alloca %struct.S*, align 8
+  %bar.addr = alloca i32, align 4
+  store %struct.S* %this, %struct.S** %this.addr, align 8
+  store i32 %bar, i32* %bar.addr, align 4
+  %this1 = load %struct.S*, %struct.S** %this.addr, align 8
+  %0 = load i32, i32* %bar.addr, align 4
+  %tobool = icmp ne i32 %0, 0
+  br i1 %tobool, label %if.then, label %if.else
+
+if.then:                                          ; preds = %entry
+  %foo = getelementptr inbounds %struct.S, %struct.S* %this1, i32 0, i32 0
+  store i32 1, i32* %foo, align 4
+  br label %if.end
+
+if.else:                                          ; preds = %entry
+  %foo2 = getelementptr inbounds %struct.S, %struct.S* %this1, i32 0, i32 0
+  store i32 0, i32* %foo2, align 4
+  br label %if.end
+
+if.end:                                           ; preds = %if.else, %if.then
+  ret void
+}
+
+; CHECK:      @foo
+; CHECK:      if.end: ; preds = %if.else, %if.then
+; CHECK-NEXT:   %.sink = phi {{.*}} !dbg