[LoopDeletion] Benefit from branches by undef conditions when symbolically executing 1st iteration

We can exploit branches by `undef` condition. Frankly, the LangRef says that
such branches are UB, so we can assume that all outgoing edges of such blocks
are dead.

However, from practical perspective, we know that this is not supported correctly
in some other places. So we are being conservative about it.

Branch by undef is treated in the following way:
- If it is a loop-exiting branch, we always assume it exits the loop;
- If not, we arbitrarily assume it takes `true` value.

Differential Revision: https://reviews.llvm.org/D104689
Reviewed By: nikic

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