glsl: Rework assignments with write_masks to have LHS chan count match RHS.

It turns out that most people new to this IR are surprised when an
assignment to (say) 3 components on the LHS takes 4 components on the
RHS.  It also makes for quite strange IR output:

(assign (constant bool (1)) (x) (var_ref color) (swiz x (var_ref v) ))
(assign (constant bool (1)) (y) (var_ref color) (swiz yy (var_ref v) ))
(assign (constant bool (1)) (z) (var_ref color) (swiz zzz (var_ref v) ))

But even worse, even we get it wrong, as shown by this line of our
current step(float, vec4):

(assign (constant bool (1)) (w)
	(var_ref t)
	(expression float b2f (expression bool >=
		    (swiz w (var_ref x))(var_ref edge))))

where we try to assign a float to the writemasked-out x channel and
don't supply anything for the actual w channel we're writing.  Drivers
right now just get lucky since ir_to_mesa spams the float value across
all the source channels of a vec4.

Instead, the RHS will now have a number of components equal to the
number of components actually being written.  Hopefully this confuses
everyone less, and it also makes codegen for a scalar target simpler.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/glsl/ir_mat_op_to_vec.cpp

@@ -310,14 +310,11 @@ ir_mat_op_to_vec_visitor::do_equal_mat_mat(ir_variable *result_var,
 	 new(this->mem_ctx) ir_expression(ir_binop_any_nequal,
 					  glsl_type::bool_type, op0, op1);
 
-      ir_rvalue *const swiz =
-	 new(this->mem_ctx) ir_swizzle(cmp, i, i, i, i, columns);
-
       ir_dereference *const lhs =
 	 new(this->mem_ctx) ir_dereference_variable(tmp_bvec);
 
       ir_assignment *const assign =
-	 new(this->mem_ctx) ir_assignment(lhs, swiz, NULL, (1U << i));
+	 new(this->mem_ctx) ir_assignment(lhs, cmp, NULL, (1U << i));
 
       this->base_ir->insert_before(assign);
    }