[DA][SDA] SyncDependenceAnalysis re-write

This patch achieves two things:
1. It breaks up the `join_blocks` interface between the SDA to the DA to
   return two separate sets for divergent loops exits and divergent,
disjoint path joins.
2. It updates the SDA algorithm to run in O(n) time and improves the
   precision on divergent loop exits.

This fixes `https://bugs.llvm.org/show_bug.cgi?id=46372` (by virtue of
the improved `join_blocks` interface) and revealed an imprecise expected
result in the `Analysis/DivergenceAnalysis/AMDGPU/hidden_loopdiverge.ll`
test.

Reviewed By: sameerds

Differential Revision: https://reviews.llvm.org/D84413