[Hexagon] Standardize vector predicate load/store pseudo instructions

- Remove unused instructions: LDriq_pred_vec_V6, STriq_pred_vec_V6, and
  the 128B counterparts.
- Rename:
    LDriq_pred_V6         PS_vloadrq_ai
    LDriq_pred_V6_128B    PS_vloadrq_ai_128B
    STriq_pred_V6         PS_vstorerq_ai
    STriq_pred_V6_128B    PS_vstorerq_ai_128B

llvm-svn: 278813