[TableGen] Reorder code in ContractNodes to prevents unnecessary recursion. NFC

The code that moves CheckOpcode before CheckType/CheckChildType/RecordDwith
was running after ContractNodes started unwinding its recursion. If a
move occurs we would start a new recursion going forward
through the list again. I don't believe this can lead to any new
combines so it was just wasted work.

This patch moves the code earlier so it doesn't start a new recursion.

Author: topperc

llvm/utils/TableGen/DAGISelMatcherOpt.cpp

@@ -135,8 +135,6 @@ static void ContractNodes(std::unique_ptr<Matcher> &MatcherPtr,
       // variants.
     }
 
-  ContractNodes(N->getNextPtr(), CGP);
-
   // If we have a CheckType/CheckChildType/Record node followed by a
   // CheckOpcode, invert the two nodes.  We prefer to do structural checks
   // before type checks, as this opens opportunities for factoring on targets
@@ -156,6 +154,8 @@ static void ContractNodes(std::unique_ptr<Matcher> &MatcherPtr,
     return ContractNodes(MatcherPtr, CGP);
   }
 
+  ContractNodes(N->getNextPtr(), CGP);
+
   // If we have a MoveParent followed by a MoveChild, we convert it to
   // MoveSibling.
   if (auto *MP = dyn_cast<MoveParentMatcher>(N)) {