[m-rev.] for review: rationalise simplify_goal_call.m

[Zoltan Somogyi]

I am doing this as cleanup before working on strength reduction
for divisions and shifts. Since it touches so much of simplify_goal_call.m,
I am attaching the updated version of that file.

For review by anyone, but since I would like to build on this soon,
I intend to commit it if noone reviews it in the next couple of days.

BTW, I noticed that the simplify_goal_call.m has two related transformations
that almost work at cross purposes.

- The first replaced calls such as @<(X, Y) with compare(R, X, Y), R = (<).
- The second replaced calls such as compare(R, X, Y) with

  if builtin_compound_eq(X, Y) then R = (=) else
  if builtin_compound_lt(X, Y) then R = (<) else R = (>)

  but only on the Erlang backend, since that is the only one
  that has builtins for comparing entire structured terms.

The first yields a speedup on non-Erlang platforms because it inlines
the definition of @< (which itself does compare(R, X, Y), R = (<)),
and the second yields a speedup on Erlang because (I presume)
it eliminates a whole lot of crossings of the Mercury/Erlang boundary.

However, the first transformation is a PESSIMIZATION for Erlang.
The obvious thing to do on that platform is to replace @<(X, Y) with builtin_compound_lt(X, Y). (NOTE: I think the bad structure of
simplify_goal_call.m up to now is probably why noone has noticed
the problem until now.)

Peter, since I think you know the Erlang backend best: do you think
such a change would be useful, and if so, if I implement it, will you
test it? (I cannot test it myself.)

Zoltan.
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