[SCEV] Look at backedge dominating conditions (re-land r233447).

Summary:
This change teaches ScalarEvolution::isLoopBackedgeGuardedByCond to look
at edges within the loop body that dominate the latch.  We don't do an
exhaustive search for all possible edges, but only a quick walk up the
dom tree.

This re-lands r233447.  r233447 was reverted because it caused massive
compile-time regressions.  This change has a fix for the same issue.

llvm-svn: 233829