[Value][InstCombine] Fix one-use checks in PHI-of-op -> Op-of-PHI[s] transforms to...

[Value][InstCombine] Fix one-use checks in PHI-of-op -> Op-of-PHI[s] transforms to be one-user checks

As FIXME said, they really should be checking for a single user,
not use, so let's do that. It is not *that* unusual to have
the same value as incoming value in a PHI node, not unlike
how a PHI may have the same incoming basic block more than once.

There isn't a nice way to do that, Value::users() isn't uniqified,
and Value only tracks it's uses, not Users, so the check is
potentially costly since it does indeed potentially involes
traversing the entire use list of a value.