[MustExecute] Forward iterate over conditional branches

Summary:
If a conditional branch is encountered we can try to find a join block
where the execution is known to continue. This means finding a suitable
block, e.g., the immediate post dominator of the conditional branch, and
proofing control will always reach that block.

This patch implements different techniques that work with and without
provided analysis.

Reviewers: uenoku, sstefan1, hfinkel

Subscribers: hiraditya, bollu, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68933