glsl: Convert mix() to use a new ir_triop_lrp opcode.

Many GPUs have an instruction to do linear interpolation which is more
efficient than simply performing the algebra necessary (two multiplies,
an add, and a subtract).

Pattern matching or peepholing this is more desirable, but can be
tricky.  By using an opcode, we can at least make shaders which use the
mix() built-in get the more efficient behavior.

Currently, all consumers lower ir_triop_lrp.  Subsequent patches will
actually generate different code.

v2 [mattst88]:
   - Add LRP_TO_ARITH flag to ir_to_mesa.cpp. Will be removed in a
     subsequent patch and ir_triop_lrp translated directly.
v3 [mattst88]:
   - Move changes from the next patch to opt_algebraic.cpp to accept
     3-src operations.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>

src/glsl/ir_validate.cpp

@@ -468,6 +468,12 @@ ir_validate::visit_leave(ir_expression *ir)
       assert(ir->operands[1]->type == glsl_type::uint_type);
       break;
 
+   case ir_triop_lrp:
+      assert(ir->operands[0]->type->base_type == GLSL_TYPE_FLOAT);
+      assert(ir->operands[0]->type == ir->operands[1]->type);
+      assert(ir->operands[2]->type == ir->operands[0]->type || ir->operands[2]->type == glsl_type::float_type);
+      break;
+
    case ir_quadop_vector:
       /* The vector operator collects some number of scalars and generates a
        * vector from them.