[Hexagon] Cleanup and standardize vector load/store pseudo instructions

Remove the following single-vector load/store pseudo instructions, use real
instructions instead:
  LDriv_pseudo_V6         STriv_pseudo_V6
  LDriv_pseudo_V6_128B    STriv_pseudo_V6_128B
  LDrivv_indexed          STrivv_indexed
  LDrivv_indexed_128B     STrivv_indexed_128B

Rename the double-vector load/store pseudo instructions, add unaligned
counterparts:

  -- old --               -- new --            -- unaligned --
  LDrivv_pseudo_V6        PS_vloadrw_io        PS_vloadrwu_io
  LDrivv_pseudo_V6_128B   PS_vloadrw_io_128B   PS_vloadrwu_io_128B
  STrivv_pseudo_V6        PS_vstorerw_io       PS_vstorerwu_io
  STrivv_pseudo_V6_128B   PS_vstorerw_io_128   PS_vstorerwu_io_128

llvm-svn: 278564