Remove the algebraic simplifier

[athas]

I have done some more profiling and found that the algebraic simplifier is still a huge cost sink. Worse, it is already so restricted that it barely ever does anything. Thus, I propose that we simply remove it entirely, and replace it with a very simple pattern-based rule that can handle the few bounds checks that are currently removed by the simplifier. In the future, we can bring it back in a better and faster form (it's also wrong in exotic cases right now).

As sample motivation, disabling the algebraic simplifier cuts the compile time of LocVolCalib (with incremental flattening) from over 9 seconds to about 5 seconds. Other benchmarks are similar.

[athas]

I have looked at bit at what it actually does for us at the moment. If we simply disable the algebraic simplifier, then a bunch of programs will need unsafe. The vast majority of cases are:

• A loop or map over iota n where we are using the element to index an array of n elements.
• A nested iota n where we must know that n is non-negative (and this is in fact guaranteed by some outer loops).

It should not be hard to create a much simpler and more efficient replacement that can handle just these cases.

[athas]

I did not remove the algebraic simplifier, but I moved its usage into a separate pass that is only run once. The improvement in compiler speed is significant. The simplifier itself will remain for now, but I still do not like it.