swift234

[simplify-cfg] When moving cond_fail to preds, do not bail early.

This commit fixes a bug where we were not checking that all predecessors had the
cond_fail block as its only successor. This occured since we were bailing early
when we saw a constant. So if we saw a predecessor with a constant before a
predecessor that had multiple successors, we would optimize even though we would
be introducing an extra cond_fail along a path.

I added a new utility pass to test this code since so much is going on in
SimplifyCFG that it is difficult to construct a test case running the full
pass.

Really this code should be in a different pass (properly SIL Code Motion TBH).
But for now, this commit just fixes the bug.

rdar://26904047