[Hexagon] Improve HVX instruction selection (bitcast, vsplat)

There was some unfortunate interaction between VSPLAT and BITCAST
related to the selection of constant vectors (coming from selecting
shuffles). Introduce VSPLATW that always splats a 32-bit word, and
can have arbitrary result type (to avoid BITCASTs of VSPLAT).
Clean up the previous selection of BITCAST/VSPLAT.

llvm-svn: 330471