Fix the partial backtrace in a recursive inferior compiled with -fomit-frame-pointer.

- Removes the block in UnwindLLDB::AddOneMoreFrame that tests for a bad stack setup,
since it is neither correct (tests the FP GPR), complete (doesn't consider multi-frame
cycles), nor reachable (the construction of RegisterContextLLDB will fail in the case
where either of the two (why just two?) previous frames have the same canonical frame
address as the frame that we propose adding to the stack).