Introduce a simple canonicalization of affine_apply that drops unused dims and

symbols.

Included with this is some other infra:
 - Testcases for other canonicalizations that I will implement next.
 - Some helpers in AffineMap/Expr for doing simple walks without defining whole
   visitor classes.
 - A 'replaceDimsAndSymbols' facility that I'll be using to simplify maps and
   exprs, e.g. to fold one constant into a mapping and to drop/renumber unused dims.
 - Allow index (and everything else) to work in memref's, as we previously
   discussed, to make the testcase easier to write.
 - A "getAffineBinaryExpr" helper to produce a binop when you know the kind as
   an enum.

This line of work will eventually subsume the ComposeAffineApply pass, but it is no where close to that yet :-)

PiperOrigin-RevId: 227852951