[LLVM][Uniformity] Fix error causing assert on some irreducible control flow

As the test case demonstrates, it is possible for a block to be identified as a
join point while not being the header of a reducible cycle. To address this,
when searching for the outermost cycle made divergent by branch outside it, we
first check for an irreducible outermost cycle before checking if the parent is
reducible.

Reviewed By: sameerds

Differential Revision: https://reviews.llvm.org/D158014