[VPlan] Make various tryTo* helpers private and mark as const (NFC).

The individual tryTo* helpers do not need to be public. Also, the
builder contained two consecutive public: sections, which is not
necessary. Moved the remaining public methods after the constructor.

Also make some of the tryTo* helpers const.

Reviewers: gilr, rengolin, Ayal, hsaito

Reviewed by: gilr

Differential Revision: https://reviews.llvm.org/D78288