[llvm-reduce] Refactor global variable delta pass

[llvm-reduce] Refactor global variable delta pass

The limitation of the current pass that it skips initializer-less GV's
seems arbitrary, in all the reduced cases i (personally) looked at,
the globals weren't needed, yet they were kept.

So let's do two things:
1. allow reducing initializer-less globals
2. before reducing globals, reduce their initializers, much like we do function bodies