[NFC][VPlan] Wrap PlainCFGBuilder with an anonymous namespace.

[a.elovikov]

Summary

It's internal to the VPlanHCFGBuilder and should not be visible outside of its
translation unit.

[fhahn]

LGTM, thanks

[dcaballe]

Thanks for the patch, Andrei!

lib/Transforms/Vectorize/VPlanHCFGBuilder.cpp

 #include "VPlanHCFGBuilder.h"
 #include "LoopVectorizationPlanner.h"
 #include "llvm/Analysis/LoopIterator.h"
 
 #define DEBUG_TYPE "loop-vectorize"
 
 using namespace llvm;
 
+namespace {
 // Class that is used to build the plain CFG for the incoming IR.
 class PlainCFGBuilder {
 private:
   // The outermost loop of the input loop nest considered for vectorization.
   Loop *TheLoop;
 
   // Loop Info analysis.
   LoopInfo *LI;

 
 public:
   PlainCFGBuilder(Loop *Lp, LoopInfo *LI, VPlan &P)
       : TheLoop(Lp), LI(LI), Plan(P) {}
 
   // Build the plain CFG and return its Top Region.
   VPRegionBlock *buildPlainCFG();
 };
+} // anonymous namespace
 
 // Return true if \p Inst is an incoming Instruction to be ignored in the VPlan
 // representation.
 static bool isInstructionToIgnore(Instruction *Inst) {
   return isa<BranchInst>(Inst);
 }
 
 // Set predecessors of \p VPBB in the same order as they are in \p BB. \p VPBB