Migrate the peephole optimization (+ ... (* -1 ...)) -> (- ... ...)
to a dedicated stage of peephole optimizations in FOL, so it starts
applying to backends other than asm.js.
Migrate explanation of why I have a separate algebraic simplification
pass after all.
Augment CSE to migrate constants to the front to + and * to make rules
easier to write.  This touched a lot of tests for inessential reasons.
Migrate comment about more optimization to do into a more relevant location.