[m-rev.] for review: Enable coerce casts in backends.

[Julien Fischer]

On Thu, 1 Apr 2021, Peter Wang wrote:

> compiler/call_gen.m:
> compiler/interval.m:
> compiler/ml_call_gen.m:
>    As above.
>
> tests/hard_coded/subtype_pack.m:
>    Enable use of coerce in test case.

That's fine.

Julien.