[PredicateInfo] Order instructions in different BBs by DFSNumIn.

Using OrderedInstructions::dominates as comparator for instructions in
BBs without dominance relation can cause a non-deterministic order
between such instructions. That in turn can cause us to materialize
copies in a non-deterministic order. While this does not effect
correctness, it causes some minor non-determinism in the final generated
code, because values have slightly different labels.

Without this patch, running -print-predicateinfo on a reasonably large
module produces slightly different output on each run.

This patch uses the dominator trees DFSInNum to order instruction from
different BBs, which should enforce a deterministic ordering and
guarantee that dominated instructions come after the instructions that
dominate them.

Reviewers: dberlin, efriedma, davide

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D48230

llvm-svn: 335150