substitute(): convert switch() into cascade of ifs

Again, better code generation that way (and I'd like to use likely()
here); it *is* in a very hot loop.

Reorder the TOKEN_... a bit (move TOKEN_UNTAINT up, so that it's less than
TOKEN_MACRO_ARGUMENT) to get the default (and by far the most common case)
via single comparison.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Chris Li <sparse@chrisli.org>