glsl: allow any l-value of an input variable as interpolant in interpolateAt*

The intended rule has been clarified in GLSL 4.60, Section 8.13.2
(Interpolation Functions):

   "For all of the interpolation functions, interpolant must be an l-value
    from an in declaration; this can include a variable, a block or
    structure member, an array element, or some combination of these.
    Component selection operators (e.g., .xy) may be used when specifying
    interpolant."

For members of interface blocks, var->data.must_be_shader_input must be
determined on-the-fly after lowering interface blocks, since we don't want
to disable varying packing for an entire block just because one input in it
is used in interpolateAt*.

v2: keep setting must_be_shader_input in ast_function (Ian)
v3: follow the relaxed rule of GLSL 4.60
v4: only apply the relaxed rules to desktop GL
    (the ES WG decided that the relaxed rules may apply in a future version
     but not retroactively; see also
     dEQP-GLES31.functional.shaders.multisample_interpolation.interpolate_at_centroid.negative.*)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101378
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com> (v1)
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>

src/compiler/glsl/lower_named_interface_blocks.cpp

@@ -115,6 +115,7 @@ public:
    void run(exec_list *instructions);
 
    virtual ir_visitor_status visit_leave(ir_assignment *);
+   virtual ir_visitor_status visit_leave(ir_expression *);
    virtual void handle_rvalue(ir_rvalue **rvalue);
 };
 
@@ -238,6 +239,23 @@ flatten_named_interface_blocks_declarations::visit_leave(ir_assignment *ir)
    return rvalue_visit(ir);
 }
 
+ir_visitor_status
+flatten_named_interface_blocks_declarations::visit_leave(ir_expression *ir)
+{
+   ir_visitor_status status = rvalue_visit(ir);
+
+   if (ir->operation == ir_unop_interpolate_at_centroid ||
+       ir->operation == ir_binop_interpolate_at_offset ||
+       ir->operation == ir_binop_interpolate_at_sample) {
+      const ir_rvalue *val = ir->operands[0];
+
+      /* This disables varying packing for this input. */
+      val->variable_referenced()->data.must_be_shader_input = 1;
+   }
+
+   return status;
+}
+
 void
 flatten_named_interface_blocks_declarations::handle_rvalue(ir_rvalue **rvalue)
 {