When turning a jump to a return into a return, we need to clear 
EDGE_CROSSING of the fallthru edge to prevent a checking failure.

I considered not applying the transformation when the edge has 
EDGE_CROSSING set, but it still seems like we ought to eliminate the 
unnecessary jump in that case.


Bootstrapped and regression tested on x86_64.  Also did a profiled 
bootstrap and regression test on x86_64.


Installing on the trunk.


Jeff