[SimpifyCFG] Remove recursion from FoldCondBranchOnPHI. NFCI.

Avoid stack overflow errors on systems with small stack sizes
by removing recursion in FoldCondBranchOnPHI.

This is a simple change as the recursion was only iteratively
calling the function again on the same arguments.
Ideally this would be compiled to a tail call, but there is
no guarantee.

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D107803