Finish adding the individual instruction tests to the x86 unwinder

unittests.  If I have time, I'd like to see if I can write some
tests of the eh_frame augmentation which is a wholly separate code
path (it seems like maybe it should be rolled into the main instruction
scanning codepath, to be honest, and operate on the generated
UnwindPlan instead of bothering with raw instructions at all).  

Outside the eh_frame augmentation, I'm comfortable that this unwind
generator is being tested well now.

llvm-svn: 283186