nir/dce: perform DCE for unlooped instructions in a single pass

nir/dce: perform DCE for unlooped instructions in a single pass

It's unnecessary to iterate twice for instructions outside loops.

Compile-time (nir_opt_dce):
Difference at 95.0% confidence
	-630.64 +/- 6.18761
	-27.0751% +/- 0.223134%
	(Student's t, pooled s = 7.30785)

Compile-time (entire run):
Difference at 95.0% confidence
	-749.54 +/- 48.8272
	-1.82644% +/- 0.117838%
	(Student's t, pooled s = 57.6672)

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7691>