Use a data structure better suited for large sets in SimplificationTracker.

Summary:
D44571 changed SimplificationTracker to use SmallSetVector to keep phi nodes. As a result, when the number of phi nodes is large, the build time performance suffers badly. When building for power pc, we have a case where there are more than 600.000 nodes, and it takes too long to compile.

In this change, I partially revert D44571 to use SmallPtrSet, which does an acceptable job with any number of elements. In the original patch, having a deterministic iteration order was mentioned as a motivation, however I think it only applies to the nodes already matched in MatchPhiSet method, which I did not touch.

Reviewers: bjope, skatkov

Reviewed By: bjope, skatkov

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D54007

llvm-svn: 346710