Hello,
Is it a convenient way except of somehow iterating from one block to another to find from which block I have initial incoming value and from which I have a changing incoming value (this is only for phi nodes with two incoming blocks)? Dominator tree requires to know one block and one block edge…this is not what I want. Is it valid to take two first instructions from two incoming blocks and see what dominates what?
You can use Loop::contains() to figure out whether a basic block is inside a loop. I think that's what you're looking for?
-Eli
I would compare depths, incoming block still might be inside a loop (nested loops).
You could also just check to see if the block belongs to the same loop as the blocks in PHI.
Dominator tree does not require this, actually 
It will happily take two blocks?
Yes, seems that this is the most convenient one. Thank you!