Reapply r287232 from upstream llvm trunk (by Daniil Fukalov):
[SCEV] limit recursion depth of CompareSCEVComplexity Summary: CompareSCEVComplexity goes too deep (50+ on a quite a big unrolled loop) and runs almost infinite time. Added cache of "equal" SCEV pairs to earlier cutoff of further estimation. Recursion depth limit was also introduced as a parameter. Reviewers: sanjoy Subscribers: mzolotukhin, tstellarAMD, llvm-commits Differential Revision: https://reviews.llvm.org/D26389
Pull in r296992 from upstream llvm trunk (by Sanjoy Das):
[SCEV] Decrease the recursion threshold for CompareValueComplexity Fixes PR32142. r287232 accidentally increased the recursion threshold for CompareValueComplexity from 2 to 32. This change reverses that change by introducing a separate flag for CompareValueComplexity's threshold.
The latter revision fixes the excessive compile times for skein_block.c.