[PATCH]: Fix SSA constant propagation

[law@redhat.com]

  In message < 87elpx2xsi.fsf@cgsoftware.com >you write:
  > Contaminate it with what?
  > You are simply doing a SET_SRC (x) = constant.
  > This is incorrect.
  > At the very least, you should use a validate function, since it's not
  > guaranteed to be valid.
  > 
  > Unless you plan on introducing a middle rtl where (set (reg:si 57)
  > (any constant)) is always valid?
This is precisely what I'm planning.

jeff