Subzero: Improve performance by using llvm::SmallVector<>.

src/IceDefs.h

@@ -30,6 +30,7 @@
 #include "llvm/ADT/ilist.h"
 #include "llvm/ADT/ilist_node.h"
 #include "llvm/ADT/SmallBitVector.h"
+#include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/Support/Casting.h"
 #include "llvm/Support/ELF.h"

src/IceOperand.h

@@ -362,7 +362,8 @@ public:
 
 private:
   typedef std::pair<InstNumberT, InstNumberT> RangeElementType;
-  typedef std::vector<RangeElementType> RangeType;
+  // Assume a common case of 2 or fewer segments per live range.
+  typedef llvm::SmallVector<RangeElementType, 2> RangeType;
   RangeType Range;
   RegWeight Weight;
   // TrimmedBegin is an optimization for the overlaps() computation.