Partially revert a patch from Ashok Thirumurthi in r191430.

The original code was not completely correct, but a form of
this check is necessary to avoid an infinite recursion on
some unwind cases where a function unwinds to itself with the
same CFA.  Ashok thought the recursion would be caught in
RegisterContextLLDB but this one isn't - we still need it here.
<rdar://problem/15664282> 

llvm-svn: 197761