[llvm][Uniformity] correctly use a vector as a set by uniqifying elements

The search for temporal divergence needs to determine a dominance frontier
defined for a cycle. The implementation uses a temporary vector to store a set
of newly discovered successors. Failing to uniqify the elements in this vector
causes a very large regression in compile time due to an exponential number of
redundant visits.

This fixes github issue #61123

Reviewed By: foad

Differential Revision: https://reviews.llvm.org/D145216