*FINALLY* Fix a really nasty nondeterministic bug that has been haunting us

since May 1st.  In this code, the pred iterator was being invalidated sometimes
causing the wrong entries to be added to PHI nodes.

The fix for this is to defererence and safe the *PI value before we hack on
branch instructions, which changes use/def chains, which SOMETIMES invalidates
the iterator.

llvm-svn: 14278