[SCEV] Refactor out isHighCostExpansion. NFCI.

Summary:
Move isHighCostExpansion from IndVarSimplify to SCEVExpander.  This
exposed function will be used in a subsequent change.

Reviewers: bogner, atrick

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8995

llvm-svn: 234844