nir/dce: replace instruction worklist with ssa def bitset

nir/dce: replace instruction worklist with ssa def bitset

Instead of a keeping a worklist of live instructions, use a bitset of live
ssa defs and iterate over instructions in reverse.

Compile-time (nir_opt_dce):
Difference at 95.0% confidence
	-931.911 +/- 4.41383
	-26.0263% +/- 0.105781%
	(Student's t, pooled s = 5.21293)

Compile-time (overall):
Difference at 95.0% confidence
	-882.245 +/- 28.3492
	-2.08541% +/- 0.0665121%
	(Student's t, pooled s = 33.4818)

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>