Only inject thread barriers where threads disagree about memory

[abadams]

InjectThreadBarriers visited Blocks pairwise, so it put a barrier at every join in a sequence of statements regardless of whether the two halves touched any of the same memory. Take the whole sequence at once and compare each statement's footprint against everything since the last barrier, emitting one only for a genuine read-after-write, write-after-read, or write-after-write.

A barrier only fences the memory spaces named in its mask, so it only clears the footprint of those spaces. Accesses in a space it did not fence are still waiting for a barrier of their own.

The footprint is keyed on the storage a Func ends up in rather than its name. ExtractSharedAndHeapAllocations pools allocations with disjoint lifetimes onto the same memory, so two differently named Funcs can be the same bytes at different times, and the hazard between them is real. allocate_funcs takes its argument by value, because the grouping is now asked for twice and sorting the caller's vector would make the second answer depend on the first.

Textures are counted too. They are reached through image_load and image_store rather than Load and Store, so walking for the latter alone would miss a Func stored in MemoryType::GPUTexture entirely.

Across the correctness tests this drops the number of barriers emitted from 259 to 181.

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.05%. Comparing base (dd9ecf9) to head (1067c09).

Files with missing lines	Patch %	Lines
src/FuseGPUThreadLoops.cpp	91.66%	3 Missing and 4 partials ⚠️

Additional details and impacted files

@@                     Coverage Diff                     @@
##           abadams/block_to_vector    #9390      +/-   ##
===========================================================
- Coverage                    70.11%   70.05%   -0.07%     
===========================================================
  Files                          261      261              
  Lines                        79190    79355     +165     
  Branches                     19305    19336      +31     
===========================================================
+ Hits                         55524    55591      +67     
- Misses                       17893    17920      +27     
- Partials                      5773     5844      +71     

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