[BasicBlock] fix looping in getPostdominatingDeoptimizeCall

Blindly following unique-successors chain appeared to be a bad idea.
In a degenerate case when block jumps to itself that goes into endless loop.

Discovered this problem when playing with additional changes,
managed to reproduce it on existing LoopPredication code.

Fix by checking a "visited" set while iterating through unique successors.

Reviewed By: skatkov

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72908