[DebugInfo][Mem2Reg] Assign uninitialized values with annotated locs

In PromoteMem2Reg, we perform a DFS over the CFG and track, for each alloca,
its incoming value and its associated incoming DebugLoc, both of which are
taken from stores to that alloca; these values and DebugLocs are propagated
to PHI nodes when new blocks are reached. In the event that for one
incoming edge no store instruction has been seen, we propagate an UndefValue
and an empty DebugLoc to the PHI.

This is a perfectly valid occurrence, and assigning an empty DebugLoc to the
PHI is the correct course of action; therefore, we should pass an annotated
DebugLoc instead, so that in DebugLoc coverage tracking we correctly do not
expect a valid DebugLoc to be present; we generally mark allocas as having
CompilerGenerated locations, so I've chosen to use the same annotation to
represent the uninitialized value of that alloca.

This change is NFC outside of DebugLoc coverage tracking builds.

Author: SLTozer

llvm/lib/Transforms/Utils/PromoteMemoryToRegister.cpp

@@ -849,9 +849,12 @@ void PromoteMem2Reg::run() {
   for (unsigned i = 0, e = Allocas.size(); i != e; ++i)
     IncomingVals.init(i, UndefValue::get(Allocas[i]->getAllocatedType()));
 
-  // When handling debug info, treat all incoming values as if they have unknown
-  // locations until proven otherwise.
+  // When handling debug info, treat all incoming values as if they have
+  // compiler-generated (empty) locations, representing the uninitialized
+  // alloca, until proven otherwise.
   IncomingLocs.resize(Allocas.size());
+  for (unsigned i = 0, e = Allocas.size(); i != e; ++i)
+    IncomingLocs.init(i, DebugLoc::getCompilerGenerated());
 
   // The renamer uses the Visited set to avoid infinite loops.
   Visited.resize(F.getMaxBlockNumber(), false);