[SCEV] Add a threshold to restrict number of mul operands to be inlined into SCEV

This is to avoid inlining too many multiplication operands into a SCEV, which could 
take exponential time in the worst case.

Reviewers: Sanjoy Das, Mehdi Amini, Michael Zolotukhin

Differential Revision: https://reviews.llvm.org/D25794

llvm-svn: 284784