i965/fs: Properly precolor payload registers on GEN5 in SIMD16

For GEN6 SIMD16 mode, we have to 2-align all the registers, so we only have
the even-numbered ones.  This means that we have to divide the register
number by 2 when we precolor.  This wasn't a problem before because we were
setting up the interference between ra_node registers wrong.  This will be
fixed in the next commit.

Signed-off-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>

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

@@ -390,7 +390,16 @@ fs_visitor::setup_payload_interference(struct ra_graph *g,
        * The alternative would be to have per-physical-register classes, which
        * would just be silly.
        */
-      ra_set_node_reg(g, first_payload_node + i, i);
+      if (brw->intelScreen->devinfo->gen <= 5 && dispatch_width == 16) {
+         /* We have to divide by 2 here because we only have even numbered
+          * registers.  Some of the payload registers will be odd, but
+          * that's ok because their physical register numbers have already
+          * been assigned.  The only thing this is used for is interference.
+          */
+         ra_set_node_reg(g, first_payload_node + i, i / 2);
+      } else {
+         ra_set_node_reg(g, first_payload_node + i, i);
+      }
    }
 }