Fix global effect computation with -O flags

[kripken]

We tested --generate-global-effects --vacuum and such, but not
--generate-global-effects -O3 or the other -O flags. Unfortunately, our
targeted testing missed a bug because of that. Specifically, we have special
logic for -O flags to make sure the passes they expand into run with the
proper opt and shrink levels, but that logic happened to also interfere with
global effect computation. It would also interfere with allowing GUFA info
or other things to be stored on the side, which we've proposed. This PR
fixes that + future issues.

The fix is to just allow a pass runner to execute more than once. We thought
to avoid that and assert against it to keep the model "hermetic" (you create
a pass runner, you run the passes, and you throw it out), which feels nice in
a way, but it led to the bug here, and I'm not sure it would prevent any other
ones really. It is also more code. It is simpler to allow a runner to execute more
than once, and add a method to clear it. With that, the logic for -O3 execution
is both simpler and does not interfere with anything but the opt and shrink
level flags: we create a single runner, give it the proper options, and then keep
using that runner + those options as we go, normally.

[tlively]

If we put an explicit (export "pointless-work" (func $pointless-work)) here, then that and everything above it in the output would be isolated here and only the various (func $export ...) implementation would be down above that function.

[kripken]

Nice, thanks, that makes the output much cleaner.