[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.

In addition to the last revision this one fixes the bug reported on
Phabricator.

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