swift234

Fix <rdar://problem/19275047> Extraneous retains/releases of self are bad

This teaches SILGenLValue that base expressions referring to self can always
be done at +0, avoiding retain/release traffic.  This eliminates some hacks 
where init() needed special code, and also reduces r/r traffic in general.

Before the example in the radar silgen'd to:

sil hidden @_TFC1x4Base3foofS0_FT_T_ : $@cc(method) @thin (@owned Base) -> () {
bb0(%0 : $Base):
  debug_value %0 : $Base  // let self             // id: %1
  strong_retain %0 : $Base                        // id: %2
  // function_ref x.Base.__allocating_init (x.Base.Type)() -> x.Base
  %3 = function_ref @_TFC1x4BaseCfMS0_FT_S0_ : $@thin (@thick Base.Type) -> @owned Base // user: %5
  %4 = metatype $@thick Base.Type                 // user: %5
  %5 = apply %3(%4) : $@thin (@thick Base.Type) -> @owned Base // user: %7
  %6 = ref_element_addr %0 : $Base, #Base.f       // user: %7
  assign %5 to %6 : $*Base                        // id: %7
  strong_release %0 : $Base                       // id: %8
  strong_release %0 : $Base                       // id: %9
  %10 = tuple ()                                  // user: %11
  return %10 : $()                                // id: %11
}

now the %2 and %8 instructions are gone.



Swift SVN r23977