[ScopHelper] Cache ScopExpander results.

The number of SCEV expressions is usually linear in the number of IR
instructions being modeled.  However, a naive SCEV visitor is not. For
an expression like x*x, "x" will be visited twice.  If x is itself an
expression like x*x, that will be visited twice, etc, and the overall
runtime is O(2^N) in the number of SCEV expressions.

To prevent this from happening, add a cache, so we only visit each SCEV
expression once.

Not sure this is the best solution. Maybe we can instead check whether
the SCEV is scop-invariant (in which case we never need to map the
value). But we don't have a utility for that at the moment.

Differential Revision: https://reviews.llvm.org/D47087

llvm-svn: 335783