Liveness & Deadcode: fix i386-specific issue with move instructions accessing
  the x87 FP stack, which must not be eliminated.
CSEgen: harden against the same x87-specific issue + against reuse of
  values in fixed hardware registers that were destroyed by a prior
  operation.


git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14877 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02