[llvm] [SCEV] Rewrite some SCEVAdd sub-expressions using loop guards. (PR #156013)

[Nikita Popov via llvm-commits]

https://github.com/nikic approved this pull request.

LGTM, assuming no compile-time impact.

If there is impact, could try doing something like `findExistingSCEVInCache(scAddExpr, Expr->operands().drop_begin())` to avoid creating and optimizing an add expression for this lookup.

https://github.com/llvm/llvm-project/pull/156013