nir: Detect overlapping array copies
This detects copies like
foo[0].a = bar[0].a
foo[0].b = bar[0].b
foo[1].a = bar[1].a
foo[1].b = bar[1].b
expanding on the existing array copy detection. This detects some copies in shaders from Strange Brigade that we currently don't. In addition there's a patch to remove unused local variables in radv, since they can be eliminated by things like this.