[MLIR][Presburger] Support lexicographic max/min union of two PWMAFunction

[MLIR][Presburger] Support lexicographic max/min union of two PWMAFunction

This patch implements a lexicographic max/min union of two PWMAFunctions.

The lexmax/lexmin union of two functions is defined as a function defined on
the union of the input domains of both functions, such that when only one of the
functions are defined, it outputs the same as that function, and if both are
defined, it outputs the lexmax/lexmin of the two outputs. On points where
neither function is defined, the union is not defined either.

Reviewed By: arjunp

Differential Revision: https://reviews.llvm.org/D128829