[CFLAA] Fix an error which cause cfl-aa to crash the compiler

[grievejia]

Summary

The bug was caused by updating a reference to a stale DenseMap entry after the map has been resized. Grabbing the reference after the resize fixes it.

[george.burgess.iv]

LGTM -- thanks for the patch! Will commit on your behalf.

[george.burgess.iv]

r268269.

lib/Analysis/CFLAliasAnalysis.cpp

 }
 
 void CFLAAResult::scan(Function *Fn) {
   auto InsertPair = Cache.insert(std::make_pair(Fn, Optional<FunctionInfo>()));
   (void)InsertPair;
   assert(InsertPair.second &&
          "Trying to scan a function that has already been cached");
 
-  Cache[Fn] = buildSetsFrom(Fn);
+  // Note that we can't do Cache[Fn] = buildSetsFrom(Fn) here: the function call
+  // may get evaluated after operator[], potentially triggering a DenseMap
+  // resize and invalidating the reference returned by operator[]
+  auto FunInfo = buildSetsFrom(Fn);
+  Cache[Fn] = std::move(FunInfo);
+
   Handles.push_front(FunctionHandle(Fn, this));
 }
 
 void CFLAAResult::evict(Function *Fn) { Cache.erase(Fn); }
 
 /// Ensures that the given function is available in the cache, and returns the
 /// entry.
 const Optional<CFLAAResult::FunctionInfo> &