[SCEVExpander] Support cost evaluation of several SCEVs with same budget

This is a follow-up from discussion in D138412. Sometimes we want to evaluate
the cost of expansion of several SCEVs together with same budget. For example,
if one of them is a bit above cheap limit, and the second one is free, then
we still want to expand. Checking each of them with "cheap" limit is a bit more
pessimistic.

Differential Revision: https://reviews.llvm.org/D138475
Reviewed By: lebedev.ri