[InstCombine][Test] Test for fix of replacing select with Phis when branch has the same labels

An additional test that allows to check the correctness of handling the case of the same
branch labels in the dominator when trying to replace select with phi-node.

Patch By: Kirill Polushin
Differential Revision: https://reviews.llvm.org/D84006
Reviewed By: mkazantsev