r600/sfn: Don't scan the whole block for ready instructions

Limit the number of tested instructions and the number of
ready instructions that might be taken into account.

This reduces the time needed to run the scheduler significantly.

Fixes: 79ca456b48
   r600/sfn: rewrite NIR backend

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Filip Gawin <filip@gawin.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18212>
(cherry picked from commit 1f5dccb760)

.pick_status.json

@@ -10300,7 +10300,7 @@
         "description": "r600/sfn: Don't scan the whole block for ready instructions",
         "nominated": true,
         "nomination_type": 1,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": "79ca456b4837b3bc21cf9ef3c03c505c4b4909f6"
     },

src/gallium/drivers/r600/sfn/sfn_scheduler.cpp

@@ -913,7 +913,8 @@ bool BlockSheduler::collect_ready_type(std::list<T *>& ready, std::list<T *>& av
    auto i = available.begin();
    auto e = available.end();
 
-   while (i != e) {
+   int lookahead = 16;
+   while (i != e && ready.size() < 16 && lookahead-- > 0) {
       if ((*i)->ready()) {
          ready.push_back(*i);
          auto old_i = i;