[m-rev.] for review: Prevent float_regs abort on constrained polymorphic inst.

[Julien Fischer]

On Tue, 6 Mar 2018, Peter Wang wrote:

> The float_regs pass made a call to inst_expand (which does not touch an
> inst with constrained_inst_vars at the top level) was followed by
> get_arg_insts (which expects the inst to be expanded), leading to a
> compiler abort.
>
> This fixes compilation of tests/valid/constrained_poly_insts.m
> when floating point registers are used.
>
> compiler/float_regs.m:
>    Replace a call to inst_expand with
>    inst_expand_and_remove_constrained_inst_vars.
>
>    Add require_complete_switch scope to a switch.
>
> compiler/inst_util.m:
> compiler/prog_mode.m:
>    Add comments for would-be callers of inst_expand and get_arg_insts.

That's fine.

Julien.