* Throw an ICE when asked to emit a cast with sign extension.

* Throw an ICE when asked to emit a cast with sign extension.

* src/comp.c (cast_kind_of_type): Enum that specifies the kind of type
in the cast enum (unsigned, signed, pointer).
(emit_coerce): Throw an ICE when asked to emit a cast with sign
extension.
(define_cast_from_to): Return NULL for casts involving sign extension.
(define_cast_functions): Specify the kind of each type in the cast
union.