beam_block: Do optimizations in the safe order

Moving of allocation instructions upwards in the instruction
stream (in order to enable further optimizations) in beam_block,
is implemented with the assumption that if a register {x,X}
contains a valid term, then all other x register with lower
numbers than X also contain valid terms. That assumption is
true after code generation.

The beam_utils:live_opt/1 optimization, however, may invalidate
that assumption. For instance, if a receive statement exports a
variable that is used, but the return value of the receive statement
is not used, then {x,1} but not {x,0} contains a valid term at the
end of the receive statement. If the receive statement is
followed by

    {bif,self,{f,0},[],{x,0}}.
    {test_heap,NumberOfWords,2}.

moving the allocation upwards will produce

    {test_heap,NumberOfWords,2}.
    {bif,self,{f,0},[],{x,0}}.

which will cause the beam_validator pass to scream loudly that
{x,0} is not live at the test_heap instruction.

Fix the problem by doing the optimizations in reverse order.

Reported-by: Jim Engquist

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