[NFC][SCEV] SCEVExpander::isHighCostExpansionHelper(): check that we processed expression first

Summary:
As far as i can tell this is still NFC.
Initially in rL146438 it was added at the top of the function,
later rL238507 dethroned it, and rL244474 did it again.

I'm not sure if we have already checked the cost of this expansion, we should be doing that again.

Reviewers: reames, mkazantsev, wmi, sanjoy, atrick, igor-laevsky

Reviewed By: mkazantsev

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73706