glsl: Demote 'type' from ir_instruction to ir_rvalue and ir_variable.

glsl: Demote 'type' from ir_instruction to ir_rvalue and ir_variable.

Variables have types, expression trees have types, but statements don't.
Rather than have a nonsensical field that stays NULL in the base class,
just move it to where it makes sense.

Fix up a few places that lazily used ir_instruction even though they
actually knew the particular subclass.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>