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

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>