Introduce a put_tuple2 instruction

[bjorng]

Sometimes when building a tuple, there is no way to avoid an
extra move instruction. Consider this code:

make_tuple(A) -> {ok,A}.

The corresponding BEAM code looks like this:

{test_heap,3,1}.
{put_tuple,2,{x,1}}.
{put,{atom,ok}}.
{put,{x,0}}.
{move,{x,1},{x,0}}.
return.

To avoid overwriting the source register {x,0}, a move
instruction is necessary.

The problem doesn't exist when building a list:

%% build_list(A) -> [A].
{test_heap,2,1}.
{put_list,{x,0},nil,{x,0}}.
return.

Introduce a new put_tuple2 instruction that builds a tuple in a
single instruction, so that the move instruction can be eliminated:

%% make_tuple(A) -> {ok,A}.
{test_heap,3,1}.
{put_tuple2,{x,0},{list,[{atom,ok},{x,0}]}}.
return.

Note that the BEAM loader already combines put_tuple and put
instructions into an internal instruction similar to put_tuple2.
Therefore the introduction of the new instruction will not speed up
execution of tuple building itself, but it will be less work for
the loader to load the new instruction.