[IDF] Enforce the returned blocks to be sorted.
Currently the order of blocks returned by IDF::calculate can be
non-deterministic. This was discovered in several attempts to enable
SSAUpdaterBulk for JumpThreading (which led to miscompare in bootstrap between
stage 3 and stage4). Originally, the blocks were put into a priority queue with
a depth level as their key, and this patch adds a DFSIn number as a second key
to specify a deterministic order across blocks from one level.
The solution was suggested by Daniel Berlin.
Reviewers: dberlin, davide
Differential Revision: https://reviews.llvm.org/D46646