"self" when those pointers are in registers.
Previously in this case the IRInterpreter would
handle them just as if the user had typed in
"$rdi", which isn't safe because $rdi is passed
in through the argument struct.

Now we correctly break out all three cases (i.e.,
normal variables in registers, $reg, and this/self),
and handle them in a way that's a little bit easier
to read and change.

This results in more accurate printing of "this" and
"self" pointers all around.  I have strengthened the
optimized-code test case for Objective-C to ensure
that we catch regressions in this area reliably in
the future.

<rdar://problem/12693963>

llvm-svn: 169924