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

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

Reviewers: dcaballe, fhahn

Reviewed By: fhahn

Subscribers: rengolin, bollu, tschuett, llvm-commits, rkruppe

Differential Revision: https://reviews.llvm.org/D47312

llvm-svn: 333187

llvm/lib/Transforms/Vectorize/VPlanHCFGBuilder.cpp

@@ -30,6 +30,7 @@
 
 using namespace llvm;
 
+namespace {
 // Class that is used to build the plain CFG for the incoming IR.
 class PlainCFGBuilder {
 private:
@@ -74,6 +75,7 @@ public:
   // 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.