[DSE,MemorySSA] Remove short-cut to check if all paths are covered.

The post-order number early continue does not work in some cases, e.g.
if a path from EarlierAccess to an exit includes a node that dominates
EarlierAccess in a cycle.

The short-cut only has very minor impact on compile-time, so it seems
straight-forward to remove it for now:

http://llvm-compile-time-tracker.com/compare.php?from=062412e79fcfedf2cf004433e42036b0333e3f83&to=d7386016a77ce1387bdbbf360f1de157faea9d31&stat=instructions

Fixes PR47285.