[MLIR][Presburger] Factor out some functionality from LexSimplex into a LexSimplexBase

LexSimplex cannot be made to support symbols for symbolic lexmin; this requires
a second class. In preparation for upstreaming support for symbolic lexmin,
keep the part of LexSimplex that are specific to non-symbolic lexmin in LexSimplex
and move the parts that are required to a common class LexSimplexBase for both to
inherit from.

Reviewed By: Groverkss

Differential Revision: https://reviews.llvm.org/D122828