[VPlan] Turn classes with all public members into structs (NFC).

struct should be used when all members are public:
 https://llvm.org/docs/CodingStandards.html#use-of-class-and-struct-keywords

Reviewers: gilr, rengolin, Ayal, hsaito

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D77865

llvm/lib/Transforms/Vectorize/VPlanTransforms.h

@@ -19,9 +19,7 @@
 
 namespace llvm {
 
-class VPlanTransforms {
-
-public:
+struct VPlanTransforms {
   /// Replaces the VPInstructions in \p Plan with corresponding
   /// widen recipes.
   static void VPInstructionsToVPRecipes(

llvm/lib/Transforms/Vectorize/VPlanValue.h

@@ -40,7 +40,7 @@ class VPSlotTracker;
 // and live-outs which the VPlan will need to fix accordingly.
 class VPValue {
   friend class VPBuilder;
-  friend class VPlanTransforms;
+  friend struct VPlanTransforms;
   friend class VPBasicBlock;
   friend class VPInterleavedAccessInfo;
   friend class VPSlotTracker;

llvm/lib/Transforms/Vectorize/VPlanVerifier.h

@@ -28,10 +28,9 @@
 
 namespace llvm {
 
-/// Class with utility functions that can be used to check the consistency and
+/// Struct with utility functions that can be used to check the consistency and
 /// invariants of a VPlan, including the components of its H-CFG.
-class VPlanVerifier {
-public:
+struct VPlanVerifier {
   /// Verify the invariants of the H-CFG starting from \p TopRegion. The
   /// verification process comprises the following steps:
   /// 1. Region/Block verification: Check the Region/Block verification