[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