Some changes to prevent searching down the stack for saved register

values for the pc or return address register.

On ios with arm64 and a binary that has multiple functions without 
individual symbol boundaries, we end up with an assembly profile
unwind plan that says lr=<same> - that is, the link register contents
are unmodified from the caller's value.  This gets the unwinder in
a loop.  

When we're off the 0th frame, we never want to look to a caller for
a pc or return-address register value.

Add checks to ReadGPRValue and ReadRegister to prevent both the pc
and ra register values from recursing.

If this causes problems with backtraces on android, let me know or
back it out and I'll look into it -- but I think these are
straightforward and don't expect problems.

<rdar://problem/24610365> 

llvm-svn: 270162