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).