SCEV does not propagate arguments through one-input Phis for sake of LCSSA
form preservation. So whenever computeSCEVAtScope is asked about such Phi,
it will not go ahead and try to analyze its input. However it is possible that when
an this input leaves the loop through LCSSA Phi, it is a provable constant.
This patch teaches computeSCEVAtScope about this case. We can generalize it
later, but so far we can only replace LCSSA Phis with their constant loop-exiting