Sketch out affine analysis structures: AffineValueMap, IntegerValueSet,

FlatAffineConstraints, and MutableAffineMap.

All four classes introduced reside in lib/Analysis and are not meant to be
used in the IR (from lib/IR or lib/Parser/). They are all mutable, alloc'ed,
dealloc'ed - although with their fields pointing to immutable affine
expressions (AffineExpr *).

While on this, update simplifyMod to fold mod to a zero when possible.

PiperOrigin-RevId: 209618437