intel/fs/ra: Stop adding RA interference to too many SENDS nodes

We only have one node per VGRF so this was adding way too much
interference.  No idea how we didn't catch this before.

Shader-db results on Kaby Lake:

    total instructions in shared programs: 15311100 -> 15311100 (0.00%)
    instructions in affected programs: 0 -> 0
    helped: 0
    HURT: 0

    total cycles in shared programs: 355468050 -> 355543197 (0.02%)
    cycles in affected programs: 2472492 -> 2547639 (3.04%)
    helped: 17
    HURT: 20

Fixes: 014edff0 "intel/fs: Add interference between SENDS sources"
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit 096ad8a8)