[DAG] Teach DAG to also reassociate vector operations

This commit teaches DAG to reassociate vector ops, which in turn enables
constant folding of vector op chains that appear later on during custom lowering
and DAG combine.

Reviewed by Andrea Di Biagio

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199135 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/SelectionDAGNodes.h

@@ -1492,6 +1492,8 @@ public:
                        unsigned &SplatBitSize, bool &HasAnyUndefs,
                        unsigned MinSplatBits = 0, bool isBigEndian = false);
 
+  bool isConstant() const;
+
   static inline bool classof(const SDNode *N) {
     return N->getOpcode() == ISD::BUILD_VECTOR;
   }