stapbpf: fix temporary registers appearing in translator output and null ptr deref.

stapbpf: fix temporary registers appearing in translator output and null ptr deref.

* bpf-opt.cxx (reg_alloc): When replacing temp registers with hard registers in
  bpf instructions, allow for a maximum of 10 dereferences of vector "partition",
  up from 2.
  (post_alloc_cleanup): Avoid removing redundant instructions if they
  are their block's only instruction since removing them will cause a null pointer
  deref in bpf-translate.cxx (output_probe).