opaque vector types?

[DJ Delorie]

Is there an opaque vector type?  Something that can be assigned
to/from other vector types of the same size, without warning?

I'm working on a coprocessor which has separate SIMD arithmetic
operations for each data size, but only one SIMD logical operation for
all sizes.  I.e. there's four ADD insns (V8QI, V4HI, etc) , but only
one AND insn.  I'd like to use an opaque vector type for the AND
builtin, to avoid warnings.