[AArch64] fix an invalid-iterator-use bug.

[sanjoy]

Summary

In AArch64PromoteConstant::appendAndTransferDominatedUses,
InsertPts[NewPt] invalidates IPI. Therefore, `InsertPts[NewPt] =
std::move(IPI->second)` is not legal.

This was caught by running make check with
http://reviews.llvm.org/D7931.

[bkramer]

Ow, that's a subtle bug. Fix LGTM.

As a microoptimization we could erase IPI via the iterator after everything we moved everything. That would save a map lookup.

lib/Target/AArch64/AArch64PromoteConstant.cpp

private:
   /// Append Use to this list and delete the entry of IPI in InsertPts.
   static void appendAndTransferDominatedUses(Instruction *NewPt, Use &Use,
                                              InsertionPoints::iterator &IPI,
                                              InsertionPoints &InsertPts) {
     // Record the dominated use.
     IPI->second.push_back(&Use);
     // Transfer the dominated uses of IPI to NewPt
     // Inserting into the DenseMap may invalidate existing iterator.
-    // Keep a copy of the key to find the iterator to erase.
+    // Keep a copy of the key to find the iterator to erase.  Keep a copy of the
+    // value so that we don't have to dereference IPI->second.
     Instruction *OldInstr = IPI->first;
-    InsertPts[NewPt] = std::move(IPI->second);
+    Uses OldUses = std::move(IPI->second);
+    InsertPts[NewPt] = std::move(OldUses);
     // Erase IPI.
     InsertPts.erase(OldInstr);
   }
 };
 } // end anonymous namespace
 
 char AArch64PromoteConstant::ID = 0;