Fix asan container-overflow (#5619)

When the reference 'e' to stack top is popped, the call to erase(e)
reads from this now invalid reference. ASAN catches this as a container
overflow. We fix this by taking a copy of the stack top instead.

Note that this was similarly fixed in upstream LLVM as well:
9987d98370

lib/Analysis/LazyValueInfo.cpp

@@ -460,7 +460,7 @@ void LazyValueInfoCache::eraseBlock(BasicBlock *BB) {
 
 void LazyValueInfoCache::solve() {
   while (!BlockValueStack.empty()) {
-    std::pair<BasicBlock*, Value*> &e = BlockValueStack.top();
+    std::pair<BasicBlock*, Value*> e = BlockValueStack.top();
     assert(BlockValueSet.count(e) && "Stack value should be in BlockValueSet!");
 
     if (solveBlockValue(e.second, e.first)) {