[SCEV] Ensure that isHighCostExpansion takes into account what is being divided

A SCEV is not low-cost just because you can divide it by a power of 2. We need to also
check what we are dividing to make sure it too is not a high-code expansion. This helps
to not expand the exit value of certain loops, helping not to bloat the code.

The change in no-iv-rewrite.ll is reverting back to what it was testing before rL194116,
and looks a lot like the other tests in replace-loop-exit-folds.ll.

Differential Revision: https://reviews.llvm.org/D58435

llvm-svn: 355393