[LVI] Don't use dominator tree in isValidAssumeForContext()

LVI and its consumers currently have quite a bit of complexity
related to dominator tree management. However, it doesn't look
like it is actually needed...

The only use of the dominator tree is inside isValidAssumeForContext().
However, due to the way LVI queries work, it is not needed:
If we query a value for some block, we will first get the edge values
from all predecessor blocks, which also includes an intersection with
assumptions that apply to the terminator of the predecessor. As such,
we will already have processed all assumptions from predecessor blocks
(this is actually stronger than what isValidAssumeForContext() does
with a DT, because this is capable of combining non-dominating
assumptions). The only additional assumptions we need to take into
account are those in the block being queried. And we don't need a
dominator tree for that.

This patch only removes the use of DT, I will drop the machinery
around it in a followup.

Differential Revision: https://reviews.llvm.org/D76797