swift234

[rc-id] Check dominance before recursively stripping instead of vis-a-versa.

Before this change, given a CFG like the following:

bb0:
  %0 = enum $Optional<Cls>, #Optional.None!enumelt
  br bb1(%0 : $Optional<Cls>)

bb1(%1 : $Optional<Cls>):
  %2 = alloc_ref $Cls
  retain_value %2 : $Cls
  release_value %1 : $Optional<Cls>
  %3 = enum $Optional<Cls>, #Optional.Some!enumelt.1, %2 : $Cls
  cond_br undef, bb1(%3 : $Optional<Cls>), bb2

bb2:
  ...

We would pair up the retain, release even though they are from different
iterations. The reason we were doing this is that we were recursively
stripping off the Phi node before checking dominance instead of
vis-a-versa.

rdar://18367606

Swift SVN r22892