[Assignment Tracking] Fix invalidated iterator usage

[Orlando]

Summary

The iterator FirstOverlap is invalidated after the call to insert - avoid dereferencing the iterator after the call to insert.

[CarlosAlbertoEnciso]

LGTM

llvm/lib/CodeGen/AssignmentTrackingAnalysis.cpp

void addDef(const VarLocInfo &VarLoc, Instruction &Before, BasicBlock &BB,
     // Check if both ends of `f` intersect the same interval `i`.
     if (IntersectStart && IntersectEnd && FirstOverlap == LastOverlap) {
       LLVM_DEBUG(dbgs() << "- Intersect single interval @ both ends\n");
       // Shorten `i` so that there's space to insert `f`.
       //      [ f ]
       // [  -   i   -  ]
       // +
       // [ i ][ f ][ i ]
+
+      // Save values for use after inserting a new interval.
       auto EndBitOfOverlap = FirstOverlap.stop();
+      unsigned OverlapValue = FirstOverlap.value();
+
+      // Shorten the overlapping interval.
       FirstOverlap.setStop(StartBit);
       insertMemLoc(BB, Before, Var, FirstOverlap.start(), StartBit,
-                   *FirstOverlap, VarLoc.DL);
+                   OverlapValue, VarLoc.DL);
 
       // Insert a new interval to represent the end part.
-      FragMap.insert(EndBit, EndBitOfOverlap, *FirstOverlap);
-      insertMemLoc(BB, Before, Var, EndBit, EndBitOfOverlap, *FirstOverlap,
+      FragMap.insert(EndBit, EndBitOfOverlap, OverlapValue);
+      insertMemLoc(BB, Before, Var, EndBit, EndBitOfOverlap, OverlapValue,
                    VarLoc.DL);
 
       // Insert the new (middle) fragment now there is space.
       FragMap.insert(StartBit, EndBit, Base);
     } else {
       // There's an overlap but `f` may not be fully contained within
       // `i`. Shorten any end-point intersections so that we can then
       // insert `f`.