nir/copy_prop: compile-time optimizations

nir_copy_prop():

Difference at 95.0% confidence
	-5159.16 +/- 16.6755
	-53.738% +/- 0.158724%
	(Student's t, pooled s = 19.6946)

overall compile-time (radv_create_shaders()):

Difference at 95.0% confidence
	-10299.9 +/- 199.074
	-7.71911% +/- 0.14009%
	(Student's t, pooled s = 235.116)

Compile-time tested with fossils of RotTR, Strange Brigade, The Talos Principle, Dark Souls 3 and GTA V.

Tested on top of !7691 (merged)

Includes !8790 (merged)