[unittest] Exercise SCEV's udiv and udiv ceiling routines

The ceiling variant was recently added (due to the work towards D105216), and we're spending a lot of time trying to find optimizations for the expression. This patch brute forces the space of i8 unsigned divides and checks that we get a correct (well consistent with APInt) result for both udiv and udiv ceiling.

(This is basically what I've been doing locally in a hand rolled C++ program, and I realized there no good reason not to check it in as a unit test which directly exercises the logic on constants.)

Differential Revision: https://reviews.llvm.org/D106083