swift234

Enhance DCE to know that "condfail(false)" is dead.
Enhance constant propagation (a diagnostic pass) to invoke the DCE utility
when producing a condfail(false).

This fixes rdar://15729207.  Before we -emit-sil'd the testcase to:

sil @_TF1t1fFT_Si : $@thin () -> Int64 {
bb0:
  %0 = tuple ()
  %1 = integer_literal $Builtin.Int1, 0           // user: %2
  cond_fail %1 : $Builtin.Int1                    // id: %2
  %3 = integer_literal $Builtin.Int64, 8          // user: %5
  %4 = integer_literal $Builtin.Int1, 0           // user: %6
  %5 = struct $Int64 (%3 : $Builtin.Int64)        // user: %7
  cond_fail %4 : $Builtin.Int1                    // id: %6
  return %5 : $Int64                              // id: %7
}

and now we produce (if you ignore debug_value):

sil @_TF1t1fFT_Si : $@thin () -> Int64 {
bb0:
  %9 = integer_literal $Builtin.Int64, 8          // user: %10
  %10 = struct $Int64 (%9 : $Builtin.Int64)       // user: %11
  return %10 : $Int64                             // id: %11
}




Swift SVN r12490