Add change_type() directive with reduction-aware overflow checks

[alexreinking]

Adds a new .change_type() directive to Halide that takes a type S and replaces a func F of type T with:

F : T = e  ~>  F_intm : S = e' ; F : T = cast(T, F_intm)

Where e' : S is a type-adjusted version of e : T such that e = cast(T, e').

We use FuncValueBounds and lossless_cast to ensure that the replaced type will not perturb values in the pipeline. The directive is aware of reductions and will add runtime asserts to ensure that accumulations are not too long if the extent cannot be proven sufficiently short.

When constructing e', change_type might adjust the form to use widening intrinsics. For instance, the result of changing the type of f32(i8a) * f32(i8b) to Int32 would be cast<int32>(widening_mul(i8a, i8b)), which is bitwise exact.

A prototype of this was written over several sessions involving several LLMs. I probably wrote as much prompt as code was produced, and I feel that the ultimate implementation is more mine than any LLM's. The tests were written by machine outright, however.

Breaking changes

None: this is a new directive.

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)

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[codecov]

Codecov Report

❌ Patch coverage is 72.13115% with 136 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.25%. Comparing base (fc9374c) to head (db7edbe).

Files with missing lines	Patch %	Lines
src/Func.cpp	74.92%	33 Missing and 52 partials ⚠️
src/IROperator.cpp	36.66%	16 Missing and 22 partials ⚠️
src/AssociativeOpsTable.cpp	66.66%	2 Missing and 3 partials ⚠️
src/AddTypeChangeChecks.cpp	71.42%	2 Missing and 2 partials ⚠️
src/ConstantInterval.cpp	86.66%	0 Missing and 2 partials ⚠️
src/Lower.cpp	66.66%	0 Missing and 1 partial ⚠️
src/Schedule.cpp	87.50%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@                      Coverage Diff                      @@
##           alexreinking/eager-inline    #9257      +/-   ##
=============================================================
+ Coverage                      70.24%   70.25%   +0.01%     
=============================================================
  Files                            257      258       +1     
  Lines                          79122    79605     +483     
  Branches                       18957    19111     +154     
=============================================================
+ Hits                           55578    55926     +348     
- Misses                         17895    17939      +44     
- Partials                        5649     5740      +91     

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