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In some circumstances, as when inlining code, when some optimization
passes are disabled or with hand-written but semantically correct Core
Erlang or BEAM assembly, a fresh reference may be live in more than one
register:
...
{allocate_zero,2,2}.
...
{call_ext,0,{extfunc,erlang,make_ref,0}}. % Ref in [x0]
...
{move,{x,0},{y,0}}. % Ref in [x0,y0]
{move,{y,1},{x,0}}. % Ref in [y0]
...
{move,{y,0},{x,0}}. % Ref in [x0,y0]
{move,{x,0},{y,1}}. % Ref in [x0,y0,y1]
{label,5}.
{loop_rec,{f,6},{x,0}}. % Ref in [y0,y1]
...
{loop_rec_end,{f,5}}.
{label,6}.
{wait,{f,5}}.
...
Pass beam_receive expects a single live register for the ref when it
encounters the loop_rec instruction and crashes with the following
reason:
$ erlc t.S
...
crash reason: {{case_clause,
{'EXIT',
{{case_clause,[{y,1},{y,0}]},
[{beam_receive,opt_recv,5,
[{file,"beam_receive.erl"},{line,154}]},
...]}}},
...}
This commit teaches beam_receive how to use a set of registers instead
of a single one when tracking fresh references, thus avoiding the crash.
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