radv: reduce the number of loaded channels for vertex input fetches

radv: reduce the number of loaded channels for vertex input fetches

It's unnecessary to load more channels than the vertex attribute
format. The remaining channels are filled with 0 for y and z,
and 1 for w.

29077 shaders in 15096 tests
Totals:
SGPRS: 1321605 -> 1318869 (-0.21 %)
VGPRS: 935236 -> 932252 (-0.32 %)
Spilled SGPRs: 24860 -> 24776 (-0.34 %)
Code Size: 49832348 -> 49819464 (-0.03 %) bytes
Max Waves: 242101 -> 242611 (0.21 %)

Totals from affected shaders:
SGPRS: 93675 -> 90939 (-2.92 %)
VGPRS: 58016 -> 55032 (-5.14 %)
Spilled SGPRs: 172 -> 88 (-48.84 %)
Code Size: 2862740 -> 2849856 (-0.45 %) bytes
Max Waves: 15474 -> 15984 (3.30 %)

This mostly helps Croteam games (Talos/Sam2017).

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>