GlobalDCE: Convert tests to opaque pointers

The script mangled both virtual-functions-base-pointer-call.ll and
virtual-functions-derived-pointer-call.ll pretty badly, deleting the
run line and most of the block comment.

Replaced the bitcast with and addrspacecast in 2002-07-17-CastRef.ll,
based on the apparent intent of the test.

For 2003-07-01-SelfReference.ll, the script produced "call ptr ()
@getfunc" which surprisingly parses as valid.