i965/fs: Handle SAMPLEINFO consistently like other texturing instructions.

i965/fs: Handle SAMPLEINFO consistently like other texturing instructions.

Seems like this texturing opcode was missing its logical counterpart
which would prevent it from taking advantage of the SIMD lowering
infrastructure, define it and plumb it through the back-end.  At some
point we'll likely want to emit a single SAMPLEINFO message shared
among all channels irrespective of this change, but for the moment
this should be enough to get the intrinsic working in SIMD32 mode.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>