[Hexagon] Fix vector spill expansion to use proper alignment

1. Add pseudos PS_vloadrv_ai and PS_vstorerv_ai: those are now used
   for single vector registers in loadRegFromStackSlot (and store...).
2. Remove pseudos PS_vloadrwu_ai and PS_vstorerwu_ai. The alignment is
   now checked when expanding spill pseudos (both in frame lowering
   and in expand-post-ra-pseudos), and a proper instruction is generated.
3. Update MachineMemOperands when dealigning vector spill slots.
4. Return vector predicate registers in getCallerSavedRegs.