glsl: Add a new ir_builder::dotlike() function.

glsl: Add a new ir_builder::dotlike() function.

dotlike() uses ir_binop_mul for scalars, and ir_binop_dot for vectors.

When generating built-in functions, we often want to use regular
multiply for scalar signatures, and dot() for vector signatures.
ir_binop_dot only works on vectors, so we have to switch opcodes,
even if the code is otherwise identical.  dotlike() makes this easy.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>