Improve loopy codegen determinism

Use an ordered set for collecting operators to ensure deterministic
variable naming, as the variable name depends on the collection order.
(As an alternative I tried sorting the output of the collector by
string, but the stringification is very expensive on simple examples
like wave-min.)

Also, improve debuggability of any remaining bugs in loopy kernels
(such as any latent nondeterminism) by giving each assignment kernel a
separate name.