[SCEV][NFC] Share value cache between SCEVs in GroupByComplexity

Current implementation of `compareSCEVComplexity` is being unreasonable with `SCEVUnknown`s:
every time it sees one, it creates a new value cache and tries to prove equality of two values using it.
This cache reallocates and gets lost from SCEV to SCEV.

This patch changes this behavior: now we create one cache for all values and share it between SCEVs.

Reviewed By: sanjoy
Differential Revision: https://reviews.llvm.org/D40597

llvm-svn: 319880