PR#6484 and PR#6486: CSE across memory allocations can present the GC with memory roots that are illegal.
Plus: lift the previous restriction that all arithmetic ops and loads have at most one result register, this isn't true for ARM with soft floats.
Plus: code refactoring and more comments.


git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@15012 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02