glsl: Add optional parameters to the ir_factory constructor.

glsl: Add optional parameters to the ir_factory constructor.

Each ir_factory needs an instruction list and memory context in order to
be useful.  Rather than creating an object and manually assigning these,
we can just use optional parameters in the constructor.

This makes it possible to create a ready-to-use factory in one line:

   ir_factory body(&sig->body, mem_ctx);

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>