[GVNSink] Fix failing GVNSink tests in the reverse iteration bot

Summary:

The elts of ActivePreds which is defined as a SmallPtrSet are copied
into Blocks using std::copy. This makes the resultant order of Blocks
non-deterministic. We cannot simply sort Blocks as they need to match
the corresponding Values. So a better approach is to define ActivePreds
as SmallSetVector.

This fixes the following failures in
http://lab.llvm.org:8011/builders/reverse-iteration:
  LLVM :: Transforms/GVNSink/indirect-call.ll
  LLVM :: Transforms/GVNSink/sink-common-code.ll
  LLVM :: Transforms/GVNSink/struct.ll

Reviewers: dberlin, jmolloy, bkramer, efriedma

Reviewed By: dberlin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D39025

llvm-svn: 316369