aco: Optimize spilling

Doubles the speed of ACO's spiller, i.e. cutting time spent spilling by 40-50% depending on the shader. There is more potential to unlock here using C++17's std::monotonic_buffer_resource, which I'll submit in a separate MR.

Summary of the changes made:

• Massively reduces overall number of allocations
  • std::vector<CONTAINER>-like objects now are never shrunk so that the contained containers don't need to re-allocate their object storage all the time
  • Two scratch containers have been moved to spill_ctx so they don't need to re-allocate object storage for each block
  • Copying containers is avoided unless necessary
• Replaces std::map with more efficient containers
  • std::unordered_map for faster lookup
  • std::vector for efficient copying and faster iteration when slow element addition/removal is acceptable
• Avoids redundant map lookup by combining element checking and access into a single map::find call
• Applies 71aab960 to the spiller

Valgrind benchmark for one example shader (previously spending 80% of the total aco_compile_shader runtime in the spiller, now only 62%):

Commit	aco::spill runtime (cycles)	Difference to previous	Relative change (normalized)	Cumulative change
(baseline)	503861690
aco/spill: Avoid unneeded copies when iterating over maps	503057792	0,803,898	0.2%	0.2%
aco: Use std::vector for the underlying container of std::stack	502243562	0,814,230	0.2%	0.3%
aco/spill: Remove unused container	502209832	0,033,730	0.0%	0.3%
aco/spill: Replace map[] with map::insert	500808314	1,401,518	0.3%	0.6%
aco/spill: Avoid copying next_use maps more often than needed	486598328	14,209,986	2.8%	3.4%
aco/spill: Persist memory allocations of local next use maps	434403228	52,195,100	10.4%	13.8%
aco/spill: Avoid destroying local next use maps over-eagerly	408126850	26,276,378	5.2%	19.0%
aco/spill: Replace vector with vector for local_next_use	354467946	53,658,904	10.6%	29.6%
aco/spill: Prefer unordered_map over map for next use distances	327301820	27,166,126	5.4%	35.0%
aco/spill: Avoid copying current_spills when not needed	322136920	5,164,900	1.0%	36.1%
aco/spill: Reduce redundant std::map lookups	321065650	1,071,270	0.2%	36.3%
aco/spill: Replace an std::map to booleans with std::set	320742138	0,323,512	0.1%	36.3%
aco/spill: Store remat list in an std::unordered_map instead of std::map	318450040	2,292,098	0.5%	36.8%
aco/spill: Change worklist to a single integer	313510223	4,939,817	1.0%	37.8%
aco/spill: Reduce allocations in next_uses_per_block	276328251	37,181,972	7.4%	45.2%
aco/spill: Clarify use of long-lived references by adding const	276317478	0,010,773	0.0%	45.2%
aco/spill: Use unordered_map for spills_exit	269587489	6,729,989	1.3%	46.5%
aco/spill: Use std::unordered_map for spills_entry	251309451	18,278,038	3.6%	50.1%