i965: Fix regs read for FS_OPCODE_INTERP_PER_SLOT_OFFSET

Dead code elimination was eating the Y offset.

Fixes the piglit test:
spec/ARB_gpu_shader5/arb_gpu_shader5-interpolateAtOffset-nonconst

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>

src/mesa/drivers/dri/i965/brw_fs.cpp

@@ -902,6 +902,8 @@ fs_inst::regs_read(fs_visitor *v, int arg) const
       return mlen;
    } else if (opcode == SHADER_OPCODE_UNTYPED_SURFACE_READ && arg == 0) {
       return mlen;
+   } else if (opcode == FS_OPCODE_INTERPOLATE_AT_PER_SLOT_OFFSET && arg == 0) {
+      return mlen;
    }
 
    switch (src[arg].file) {