radeonsi: initialize output, temp, and address registers to "undef"

radeonsi: initialize output, temp, and address registers to "undef"

This removes "v_mov v0, 0" which typically occurs before exports.

Totals:
SGPRS: 345216 -> 344552 (-0.19 %)
VGPRS: 197684 -> 197132 (-0.28 %)
Code Size: 7390408 -> 7375376 (-0.20 %) bytes
LDS: 91 -> 91 (0.00 %) blocks
Scratch: 1842176 -> 1679360 (-8.84 %) bytes per wave

Totals from affected shaders:
SGPRS: 101336 -> 100672 (-0.66 %)
VGPRS: 53920 -> 53368 (-1.02 %)
Code Size: 2170176 -> 2155144 (-0.69 %) bytes
LDS: 2 -> 2 (0.00 %) blocks
Scratch: 1015808 -> 852992 (-16.03 %) bytes per wave

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Tom Stellard <thomas.stellard@amd.com>