[VPlan] Use VPUser to manage CondBit

VP blocks keep track of a condition, which is a VPValue. This patch
updates VPBlockBase to manage the value using VPUser, so
replaceAllUsesWith properly updates the condition bit as well.

This is required to enable VP2VP transformations and it helps with
simplifying some of the code required to manage condition bits.

Reviewed By: gilr

Differential Revision: https://reviews.llvm.org/D95382