[LVI] Remove recursion from getValueForCondition (NFCI)

Convert getValueForCondition to a worklist model instead of using
recursion.

In pathological cases getValueForCondition recurses heavily.
Stack frames are quite expensive on x86-64, and some operating
systems (e.g. Windows) have relatively low stack size limits.
Using a worklist avoids potential failures from stack overflow.

Differential Revision: https://reviews.llvm.org/D104191