beam_disasm: Resolve live registers
beam_disasm: Resolve live registers

`beam_validator:validate_live/2` expects live registers to be a
non-negative integer. This change unwraps the `{u,_}` tag for some
instructions which did not previously do this unwrapping:

- bs_get_tail/3
- bs_get_position/3
- bs_start_match4/4

erlang/otp,erlang/otp,erlang/otp,bjorng/otp,erlang/otp,erlang/otp,bjorng/otp,erlang/otp,bjorng/otp,erlang/otp,erlang/otp,bjorng/otp,bjorng/otp,bjorng/otp,bjorng/otp,erlang/otp,bjorng/otp,bjorng/otp,erlang/otp,bjorng/otp