[VPlan] Add VPBlockUtils::blocksOnly helper.

This patch adds a blocksOnly helpers which take an iterator range
over VPBlockBase * or const VPBlockBase * and returns an interator
range that only include BlockTy blocks. The accesses are casted to
BlockTy.

Reviewed By: a.elovikov

Differential Revision: https://reviews.llvm.org/D101093