glsl: Implement [iu]mulExtended() built-ins for ARB_gpu_shader5.

These built-ins have two "out" parameters, which makes implementing them
efficiently with our current compiler infrastructure difficult. Instead,
implement them in terms of the existing ir_binop_mul IR (to return the
low 32-bits) and a new ir_binop_mul64 which returns the high 32-bits.

v2: Rename mul64 -> imul_high as suggested by Ken.
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/glsl/ir.cpp

@@ -398,6 +398,7 @@ ir_expression::ir_expression(int op, ir_rvalue *op0, ir_rvalue *op1)
       this->type = glsl_type::uint_type;
       break;
 
+   case ir_binop_imul_high:
    case ir_binop_carry:
    case ir_binop_borrow:
    case ir_binop_lshift:
@@ -529,6 +530,7 @@ static const char *const operator_strs[] = {
    "+",
    "-",
    "*",
+   "imul_high",
    "/",
    "carry",
    "borrow",