Avoid heap allocation for binary search work stack.

src/IceTargetLoweringX86BaseImpl.h

 //===- subzero/src/IceTargetLoweringX86BaseImpl.h - x86 lowering -*- C++ -*-==//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
@@ skipping 4680 matching lines @@
 
   // A span is over the clusters
   struct SearchSpan {
     SearchSpan(SizeT Begin, SizeT Size, typename Traits::Insts::Label *Label)
         : Begin(Begin), Size(Size), Label(Label) {}
 
     SizeT Begin;
     SizeT Size;
     typename Traits::Insts::Label *Label;
   };
-  std::stack<SearchSpan, std::deque<SearchSpan, CfgLocalAllocator<SearchSpan>>>
-      SearchSpanStack;
+  // The stack will only grow to the height of the tree so 12 should be plenty
+  std::stack<SearchSpan, llvm::SmallVector<SearchSpan, 12>> SearchSpanStack;
   SearchSpanStack.emplace(0, CaseClusters.size(), nullptr);
   bool DoneCmp = false;
 
   while (!SearchSpanStack.empty()) {
     SearchSpan Span = SearchSpanStack.top();
     SearchSpanStack.pop();
 
     if (Span.Label != nullptr)
       Context.insert(Span.Label);
 
@@ skipping 903 matching lines @@
   }
   // the offset is not eligible for blinding or pooling, return the original
   // mem operand
   return MemOperand;
 }
 
 } // end of namespace X86Internal
 } // end of namespace Ice
 
 #endif // SUBZERO_SRC_ICETARGETLOWERINGX86BASEIMPL_H