Add stream_loads and stream_stores scheduling directives

[alexreinking]

Implements the stream_loads and stream_stores directives as proposed in #9206.

• stream_loads - ensure that loads this stage makes are streamed. It has two modes
  • Bare .stream_loads() -- stream all direct loads after inlining, excluding self-loads.
  • g.stream_loads({f1, ..., fN}) -- stream loads from the named funcs after inlining. Naming g here is a user_error.
• stream_stores - ensure that stores to this func are streamed on a per-stage basis. A stage can have this specialization only if all of its RVars are pure (meaning that no two stores can alias). If any stage-specialization requests streaming, a fence is placed after the specialization block. This is vacuously true for pure funcs, RDom-less updates, and stages without specializations.

This fully recovers performance of memcpy on my Apple M3.

Fixes #9206

Checklist

• Tests added or updated (not required for docs, CI config, or typo fixes)
• Documentation updated (if public API changed)
• Python bindings updated (if public API changed)
• Benchmarks are included here if the change is intended to affect performance.
• Commits include AI attribution where applicable (see Code of Conduct)

[alexreinking]

This implementation was super straightforward, but also super messy. It would be nice if we had an annotations system so that places that just forward those annotations don't need to be updated for every new feature.

[codecov]

Codecov Report

❌ Patch coverage is 60.77482% with 162 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@2e2336b). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/StmtToHTML.cpp	0.00%	38 Missing ⚠️
src/IRPrinter.cpp	0.00%	20 Missing and 2 partials ⚠️
src/CodeGen_LLVM.cpp	60.00%	10 Missing and 6 partials ⚠️
src/Func.cpp	60.97%	12 Missing and 4 partials ⚠️
src/IREquality.cpp	20.00%	6 Missing and 2 partials ⚠️
src/Simplify_Stmts.cpp	61.90%	5 Missing and 3 partials ⚠️
src/StorageFlattening.cpp	83.87%	4 Missing and 1 partial ⚠️
src/CodeGen_D3D12Compute_Dev.cpp	0.00%	4 Missing ⚠️
src/CodeGen_PTX_Dev.cpp	0.00%	4 Missing ⚠️
src/Derivative.cpp	0.00%	4 Missing ⚠️
... and 17 more

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9207   +/-   ##
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  Coverage        ?   69.76%           
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  Files           ?      255           
  Lines           ?    78828           
  Branches        ?    18843           
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  Hits            ?    54997           
  Misses          ?    18267           
  Partials        ?     5564           

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