ac,radv: remove the vec3 restriction with LLVM 9+

ac,radv: remove the vec3 restriction with LLVM 9+

This changes requires LLVM r356755.

32706 shaders in 16744 tests
Totals:
SGPRS: 1448848 -> 1455984 (0.49 %)
VGPRS: 1016684 -> 1016220 (-0.05 %)
Spilled SGPRs: 25871 -> 25815 (-0.22 %)
Spilled VGPRs: 122 -> 122 (0.00 %)
Scratch size: 11964 -> 11956 (-0.07 %) dwords per thread
Code Size: 55324500 -> 55301152 (-0.04 %) bytes
Max Waves: 235660 -> 235586 (-0.03 %)

Totals from affected shaders:
SGPRS: 293704 -> 300840 (2.43 %)
VGPRS: 246716 -> 246252 (-0.19 %)
Spilled SGPRs: 159 -> 103 (-35.22 %)
Scratch size: 188 -> 180 (-4.26 %) dwords per thread
Code Size: 8653664 -> 8630316 (-0.27 %) bytes
Max Waves: 60811 -> 60737 (-0.12 %)

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>