CVP: Make CVP iterate in an order that maximizes reuse of LVI cache

Summary:
After the DFS order change for LVI, i have a few testcases that now
take forever.

The TL;DR - This is mainly due to the overdefined cache, but that
requires predicateinfo to fix[1]

In order to maximize reuse of the LVI cache for now, change the order
we iterate in.

This reduces my testcase from 5 minutes to 4 seconds.

I have verified cases like gmic do not get slower.

I am playing with whether the order should be postorder or idf.

[1] In practice, overdefined anywhere should be overdefined
everywhere, so this cache should be global.  That also fixes this bug.
The problem, however, is that LVI relies on this cache being filled in
per-block because it wants different values in different blocks due to
precisely the naming issue that predicateinfo fixes.  With
predicateinfo, making the cache global works fine on individual
passes, and also resolves this issue.

Reviewers: davide, sanjoy, chandlerc

Subscribers: llvm-commits, djasper

Differential Revision: https://reviews.llvm.org/D29679

llvm-svn: 294398