[Hexagon] Define certain aliases for vector instructions

Specifically:
  Vd = #0   -> Vd = vxor(Vd, Vd)
  Vdd = #0  -> Vdd.w = vsub(Vdd.w, Vdd.w)
  Vdd = Vss -> Vdd = vcombine(Vss.H, Vss.L)

llvm-svn: 267901