This patch aims to improve compile time performance by increasing
the SCEV vector size in LoopStrengthReduce. It is observed that
the BaseRegs vector size is 4 in most cases,
and elements are frequently copied when it is initialized as
SmallVector<const SCEV *, 2> BaseRegs.
Our benchmark results show that the compilation time performance
improved by ~0.5%.

Patch by Wan Xiaofei.




git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174219 91177308-0d34-0410-b5e6-96231b3b80d8