- x86 back end: change code generation convention, so that instead of

- x86 back end: change code generation convention, so that instead of
  dispatchers CALLing generated code which later RETs, dispatchers
  jump to generated code and it jumps back to the dispatcher.  This
  removes two memory references per translation run and by itself
  gives a measureable performance improvement on P4.  As a result,
  there is new plumbing so that the caller of LibVEX_Translate can
  supply the address of the dispatcher to jump back to.

  This probably breaks all other targets.  Do not update.

- Administrative cleanup: LibVEX_Translate has an excessive
  number of arguments.  Remove them all and instead add a struct
  by which the arguments are supplied.  Add further comments
  about the meaning of some fields.

git-svn-id: svn://svn.valgrind.org/vex/trunk@1494