Instead allow the const to be converted each time it is const
evaluated as part of another expression. This allows an abstract const
to be used as a different type depending on the context.
As a result, abstract types may now find their way in to the IR, which
we don't want. This occurs because the compact pass treats named
expressions as used, mostly so that our snapshot tests are more
useful, and therefore does not remove them. To prevent this, we avoid
adding abstract-typed local consts to the named expressions list. This
will have no functional effect on any shaders produced by the
backends, but some unused local const declarations will no longer be
present.
Instead allow the const to be converted and each time it is const
evaluated as part of another expression. This allows an abstract const
to be used as a different type depending on the context.
A consequence of this is that abstract types may now find their way to
the validation stage, which we don't want. We therefore additionally
now ensure that the compact pass removes global constants of abstract
types. This will have no *functional* effect on shaders generated by
the backends, as the expressions belonging to the abstract consts in
the IR will not actually be used, as any usage in the input shader
will have been const-evaluated away. Certain unused const declarations
will now be removed, however, as can be seen by the effect on the
snapshot outputs.
