Take shortcuts if we're after a run-time issue

[mvdan]

As suggested by @rogpeppe, if we know we're after a run-time crash, we can take many assumptions safely that we can't if we're after compiler bugs. Some examples:

• Code that isn't being run can be removed (via code coverage)
• Statements after a panic/return can be removed (likely covered by the item above)
• Packace members (vars, consts, types) that aren't used can be removed (perhaps exclude exported ones)

Anything else?

Also note that none of these should make the tool better at reducing a program (I think?). They should only make it faster since it can get rid of many things at once instead of one by one.