[Dominators] Simplify child lists and make them deterministic

This fixes an extremely subtle non-determinism that can only be
triggered by an unfortunate alignment of passes. In my case:

- JumpThreading does large dominator tree updates
- CorrelatedValuePropagation preserves domtree now
- LICM codegen depends on the order of children on domtree nodes

The last part is non-deterministic if the update was stored in a set.
But it turns out that the set is completely unnecessary, updates are
deduplicated at an earlier stage so we can just use a vector, which is
both more efficient and doesn't destroy the input ordering.

I didn't manage to get the 240 MB IR file reduced enough, triggering
this bug requires a lot of jump threading, so landing this without a
test case.

Differential Revision: https://reviews.llvm.org/D48392

llvm-svn: 335176