[PassManager] Tuning Memory Usage of AnalysisUsage

We were using unneccessarily large initial sizes for these SmallVectors.  This was wasting around 50kb of memory for the O3 pipeline, even after the uniquing changes.  We're still using around 20kb which is a bit much, but it's definitely better.  This is about a 6% improvement in total O3 memory usage.

Note: The raw data on structure size which were used to pick these thresholds can be found in the review thread.

Differential Revision: http://reviews.llvm.org/D15244

llvm-svn: 254974