non-static loop bounds.

This is intended to help the case when one loop's bounds depend on
another loop, such as the domain

{ [i,j]: 0 <= i <= n and i <= j <= i }

In this case we want the generated code to look like

    for (i = 1; i <= n; ++i)
        for (j = i; j <= i; ++i)
            ...

rather than "for (j = 1; j <= n; ++j)" which are the static loop
bounds for j.

Additional changes that help realize this are as follows:

* Enable code generation of piecewise affine loop bounds, so that
piecewise affine constraints on the "j" loop may be expressed as
conditional expressions.

* Additionally, if "i" is marked local-parallel, we allow the "j" loop
bounds to depend on "i" as long as there are no barriers inside the
"j" loop.

Should fix https://github.com/inducer/loopy/issues/50.

in doc and tests.

Toward common subexpression elimination in subscripts: generate C (and related languages) via an AST