[MLIR] Simplex: split some basic functionality out into a SimplexBase class

This is a purely mechanical patch moving some functionality out from the
`Simplex` class out into a `SimplexBase` class. This pavees the way for
a future patch adding support for lexicographic optimization with a class
`LexSimplex`, which will inherit from `SimplexBase`. Inheriting directly
from `Simplex` would bring many additional functions that would not work in
`LexSimplex` because it operates slighty differently from `Simplex`. So We
split out only the basic functionality it needs to inherit into `SimplexBase`.

Reviewed By: Groverkss

Differential Revision: https://reviews.llvm.org/D115831