[FuncSpec] Add Phi nodes to the InstCostVisitor.

This patch allows constant folding of PHIs when estimating the user
bonus. Phi nodes are a special case since some of their inputs may
remain unresolved until all the specialization arguments have been
processed by the InstCostVisitor. Therefore, we keep a list of dead
basic blocks and then lazily visit the Phi nodes once the user bonus
has been computed for all the specialization arguments.

Differential Revision: https://reviews.llvm.org/D154852