nir: use a single set during CSE

nir: use a single set during CSE

Use a single set and ensure dominance by checking after a equivalent
instruction is found.

Besides removing the need to copy a set, this also lets us resize the set
at the start of the pass in the next commit.

ministat (CSE only):
Difference at 95.0% confidence
	-984.956 +/- 28.8559
	-6.90075% +/- 0.190231%
	(Student's t, pooled s = 26.9052)

ministat (entire run):
Difference at 95.0% confidence
	-1246.1 +/- 257.253
	-0.998972% +/- 0.205094%
	(Student's t, pooled s = 239.863)

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Co-authored-by: Daniel Schürmann <daniel@schuermann.dev>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6390>