[comb] Break out some canonicalizations into optimization pass

[teqdruid]

We are probably abusing canonicalizations to implement optimizations which should really be in some sort of optimize-comb pass. Canonizations should really just be simple transformations which aid analysis. So we should create a pass, identify which canonicalizations should be moved and move them.

I think @darthscsi agrees with me. Dunno who else does.