nir/copy_prop: use nir_{instr,if}_rewrite_{src,condition}_ssa

nir/copy_prop: use nir_{instr,if}_rewrite_{src,condition}_ssa

Compile-time (nir_copy_prop):
Difference at 95.0% confidence
	-2470.88 +/- 19.8762
	-35.7461% +/- 0.247259%
	(Student's t, pooled s = 23.4747)

Compile-time (overall):
Difference at 95.0% confidence
	-2175.72 +/- 178.786
	-1.73627% +/- 0.140826%
	(Student's t, pooled s = 211.155)

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8784>