Fix for PR3016: detect the tricky case, where there are

Fix for PR3016: detect the tricky case, where there are
unfoldable references to a PHI node in the block being folded, and disable
the transformation in that case.  The correct transformation of such PHI
nodes depends on whether BB dominates Succ, and dominance is expensive
to compute here.  (Alternatively, it's possible to check whether any
uses are live, but that's also essentially a dominance calculation.
Another alternative is to use reg2mem, but it probably isn't a good idea to
use that in simplifycfg.)

Also, remove some incorrect code from CanPropagatePredecessorsForPHIs
which is made unnecessary with this patch: it didn't consider the case
where a PHI node in BB has multiple uses.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@79174 91177308-0d34-0410-b5e6-96231b3b80d8