[VPlan] Move using namespace VPlanPatternMatch to top level (NFC).

VPlanConstruction.cpp doesn't use IR pattern matching, so using
VPlanPatternMatch can be moved to the top-level, to slightly reduce the
diff in follow-up changes.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlanConstruction.cpp

@@ -25,6 +25,7 @@
 #define DEBUG_TYPE "vplan"
 
 using namespace llvm;
+using namespace VPlanPatternMatch;
 
 namespace {
 // Class that is used to build the plain CFG for the incoming IR.
@@ -427,7 +428,6 @@ static void createLoopRegion(VPlan &Plan, VPBlockBase *HeaderVPB) {
 static void addCanonicalIVRecipes(VPlan &Plan, VPBasicBlock *HeaderVPBB,
                                   VPBasicBlock *LatchVPBB, Type *IdxTy,
                                   DebugLoc DL) {
-  using namespace VPlanPatternMatch;
   Value *StartIdx = ConstantInt::get(IdxTy, 0);
   auto *StartV = Plan.getOrAddLiveIn(StartIdx);