Make PredIteratorCache size() logically const. Do not require copying predecessors...

Make PredIteratorCache size() logically const. Do not require copying predecessors to get size.

Summary:
Every single benchmark i can run, on large and small cfgs, fully
connected, etc, across 3 different platforms (x86, arm., and PPC) says
that the current pred iterator cache is a losing proposition.

I can't find a case where it's faster than just walking preds, and in some cases, it's 5-10% slower.

This is due to copying the preds.
It also degrades into copying the entire cfg.

The one operation that is occasionally faster is the cached size.
This makes that operation faster by not relying on having the copies available.

I'm not even sure that is faster enough to be worth it. I, again, have
trouble finding cases where this takes long enough in a pass to be
worth caching compared to a million other things they could cache or
improve.

My suggestion:
We next remove the get() interface.
We do stronger benchmarking of size().
We probably end up killing this entire cache.
/

Reviewers: chandlerc

Subscribers: aemerson, llvm-commits, trentxintong

Differential Revision: https://reviews.llvm.org/D30873

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@297733 91177308-0d34-0410-b5e6-96231b3b80d8