LSR: try not to blow up solving combinatorial problems brute force.

LSR: try not to blow up solving combinatorial problems brute force.

This places limits on CollectSubexprs to constrains the number of
reassociation possibilities. It limits the recursion depth and skips
over chains of nested recurrences outside the current loop.

Fixes PR13361. Although underlying SCEV behavior is still potentially bad.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@160340 91177308-0d34-0410-b5e6-96231b3b80d8