[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