[Hexagon] Introduce PS_vsplat[ir][bhw] pseudo instructions

HVX v60 only has splats that take a 32-bit word as input, while v62+
has splats that take 8- or 16-bit value. This makes writing output
patterns that need to use a splat annoying, because the entire output
pattern needs to be replicated for various versions of HVX.
To avoid this, the patterns will always use the pseudos, and then the
pseudos will be handled using a post-ISel hook.