radv: handle unaligned vertex fetches on GFX6/GFX10

The Vulkan spec doesn't have any words for vertex attributes alignment.

Fixes a test failure on GFX6 and a GPU hang on GFX10 with:
dEQP-VK.spirv_assembly.instruction.spirv1p4.entrypoint.tess_con_pc_entry_point

vkpipeline-db results on GFX10:
Totals from affected shaders:
SGPRS: 463772 -> 472972 (1.98 %)
VGPRS: 343208 -> 343752 (0.16 %)
Spilled SGPRs: 323 -> 336 (4.02 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Code Size: 13806200 -> 14164472 (2.60 %) bytes
Max Waves: 84021 -> 83755 (-0.32 %)

Closes: mesa/mesa#2161
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>