radv: use nir_opt_copy_prop_vars and nir_opt_dead_write_vars

radv: use nir_opt_copy_prop_vars and nir_opt_dead_write_vars

Totals from affected shaders:
SGPRS: 2856 -> 2856 (0.00 %)
VGPRS: 3236 -> 3248 (0.37 %)
Spilled SGPRs: 0 -> 0 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Private memory VGPRs: 0 -> 0 (0.00 %)
Scratch size: 0 -> 0 (0.00 %) dwords per thread
Code Size: 236560 -> 233548 (-1.27 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Max Waves: 277 -> 283 (2.17 %)
Wait states: 0 -> 0 (0.00 %)

Even in the cases were we have increased VGPR use it appears
the NIR is improved significantly.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>