i965/vec4: add a horiz_offset() helper
i965/vec4: add a horiz_offset() helper

This will come in handy when we implement a simd lowering pass in a
follow-up patch.

v2: use byte_offset()

Reviewed-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler