[Hexagon] Mark vector loads as predicable, update instruction mappings

All loads of form V6_vL32b_{,cur,nt,tmp,nt_cur,nt_tmp}_{ai,pi,ppu} are
predicable on v62 (but not on v60). Mark them all as predicable in the
instruction definitions, and handle the v60 case in HII::isPredicable.

llvm-svn: 316098