[Patch][Review Requested][Compilation Time] Avoid frequent copy of elements in LoopStrengthReduce

[Murali, Sriram]

Hello,
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.

Please review.

Thanks
Sriram
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