Fix "make stage[12]-dump"

stage0-dump happened to work because in CL, (cdr nil) => nil.
Convert define-simplify uses in codegen.pco to define-codegen-simplify

There was no reason for these transformations to occur early in
compilation.  The were done in the simplify phase because that was
all we had at the time.
Move vector operator genericization to the codegen-simplify phase

And from compiler.pco to codegen-generic.pco
Convert a few more simplifies codegen-simplifies

All the remaining simplifies relate to core language features.