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

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;
}
```

Reviewers: dblaikie, gbedwell, aprantl, vsk

Reviewed By: vsk

Subscribers: vsk, JDevlieghere, llvm-commits

Tags: #debug-info

Differential Revision: https://reviews.llvm.org/D54019

llvm-svn: 346027

1 files changed, 1 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/MergedLoadStoreMotion.cpp b/llvm/lib/Transforms/Scalar/MergedLoadStoreMotion.cpp
index 3464b759280..ee21feca8d2 100644
--- a/llvm/lib/Transforms/Scalar/MergedLoadStoreMotion.cpp
+++ b/llvm/lib/Transforms/Scalar/MergedLoadStoreMotion.cpp
@@ -211,6 +211,7 @@ PHINode *MergedLoadStoreMotion::getPHIOperand(BasicBlock *BB, StoreInst *S0,
 
   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;