intel/brw: Only force g0's liveness to be the whole program if spilling

We don't actually need to extend g0's live range to the EOT message
generally - most messages that end a shader are headerless.  The main
implicit use of g0 is for constructing scratch headers.  With the last
two patches, we now consider scratch access that may exist in the IR
and already extend the liveness appropriately.

There is one remaining problem: spilling.  The register allocator will
create new scratch messages when spilling a register, which need to
create scratch headers, which need g0.  So, every new spill or fill
might extend the live range of g0, which would create new interference,
altering the graph.  This can be problematic.

However, when compiling SIMD16 or SIMD32 fragment shaders, we don't
allow spilling anyway.  So, why not use allow g0?  Also, when trying
various scheduling modes, we first try allocation without spilling.
If it works, great, if not, we try a (hopefully) less aggressive
schedule, and only allow spilling on the lowest-pressure schedule.

So, even for regular SIMD8 shaders, we can potentially gain the use
of g0 on the first few tries at scheduling+allocation.

Once we try to allocate with spilling, we go back to reserving g0
for the entire program, so that we can construct scratch headers at
any point.  We could possibly do better here, but this is simple and
reliable with some benefit.

Thanks to Ian Romanick for suggesting I try this approach.

fossil-db on Alchemist shows some more spill/fill improvements:

   Totals:
   Instrs: 149062395 -> 149053010 (-0.01%); split: -0.01%, +0.00%
   Cycles: 12609496913 -> 12611652181 (+0.02%); split: -0.45%, +0.47%
   Spill count: 52891 -> 52471 (-0.79%)
   Fill count: 101599 -> 100818 (-0.77%)
   Scratch Memory Size: 3292160 -> 3197952 (-2.86%)

   Totals from 416541 (66.59% of 625484) affected shaders:
   Instrs: 124058587 -> 124049202 (-0.01%); split: -0.01%, +0.01%
   Cycles: 3567164271 -> 3569319539 (+0.06%); split: -1.61%, +1.67%
   Spill count: 420 -> 0 (-inf%)
   Fill count: 781 -> 0 (-inf%)
   Scratch Memory Size: 94208 -> 0 (-inf%)

Witcher 3 shows a 33% reduction in scratch memory size, for example.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30319>

src/intel/compiler/brw_schedule_instructions.cpp

@@ -848,7 +848,7 @@ instruction_scheduler::setup_liveness(cfg_t *cfg)
    }
 
    int payload_last_use_ip[hw_reg_count];
-   s->calculate_payload_ranges(hw_reg_count, payload_last_use_ip);
+   s->calculate_payload_ranges(true, hw_reg_count, payload_last_use_ip);
 
    for (unsigned i = 0; i < hw_reg_count; i++) {
       if (payload_last_use_ip[i] == -1)