aco: fix vertex fetch alignment issues on GFX10 and skip unused channels at the start
A similar alignment fix for NIR->LLVM was done in !2937 (merged)
This differs from the fix in !2937 (merged): instead of always splitting it into single component loads, it tries to use MUBUF or split the load into several 1, 2 or 3 component loads first
Edited by Rhys Perry