[NFCI][SCEV] Avoid recursion in SCEVExpander::isHighCostExpansion*()

Summary:
As noted in [[ https://bugs.llvm.org/show_bug.cgi?id=45201 | PR45201 ]],
[[ https://bugs.llvm.org/show_bug.cgi?id=10090 | PR10090 ]] SCEV doesn't
always avoid recursive algorithms, and that causes issues with
large expression depths and/or smaller stack sizes.

In `SCEVExpander::isHighCostExpansion*()` case, the refactoring to avoid
recursion is rather idiomatic. We simply need to place the root expr
into a vector, and iterate over vector elements accounting for the cost
of each one, adding new exprs at the end of the vector,
thus achieving recursion-less traversal.

The order in which we will visit exprs doesn't matter here,
so we will be fine with the most basic approach of using SmallVector
and inserting/extracting from the back, which accidentally is the same
depth-first traversal that we were doing previously recursively.

Reviewers: mkazantsev, reames, wmi, ekatz

Reviewed By: mkazantsev

Subscribers: hiraditya, javed.absar, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76273