[analyzer; new edges] Ignore self-edges, not all edges with the same location.

The edge optimizer needs to see edges for, say, implicit casts (which have
the same source location as their operand) to uniformly simplify the
entire path. However, we still don't want to produce edges from a statement
to /itself/, which could occur when two nodes in a row have the same
statement location.

This necessitated moving the check for redundant notes to after edge
optimization, since the check relies on notes being adjacent in the path.

<rdar://problem/14061675>
[analyzer; new edges] Don't crash if the top-level entry edge is missing.

We previously asserted that there was a top-level function entry edge, but
if the function decl's location is invalid (or within a macro) this edge
might not exist. Change the assertion to an actual check, and don't drop
the first path piece if it doesn't match.

<rdar://problem/14070304>