Aligned splits, take 2.

[mcourteaux]

Inner loops go from 0 to factor, to help with constant bounds analysis.
2D tiled test: compute_at test overwrites the compute and storage bounds by just passing those in the schedule.

// f.split(x, xo, xi, 32);
for (xo, f.min.0, f.max.0 / 32) {
  for (xi, 0, 31) {
      let x = xo * 32 + xi 
  }
}

It's now possible to align the first iteration of the inner loop, like so:

// f.split(x, xo, xi, 32, p);
for (xo, (f.min.0 - p) / 32, (f.max.0 - p) / 32) {
  for (xi, 0, 31) {
      let x = xo * 32 + xi + p
      if (x >= f.min.0 && x <= f.max.0) { // for GuardWithIf

      }
  }
}

Replaces #9371

Breaking changes

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)

[codecov]

Codecov Report

❌ Patch coverage is 93.54839% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.23%. Comparing base (36c47dc) to head (4ff4d6e).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Func.cpp	70.83%	6 Missing and 1 partial ⚠️
src/ApplySplit.cpp	98.64%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9409      +/-   ##
==========================================
+ Coverage   70.17%   70.23%   +0.06%     
==========================================
  Files         261      261              
  Lines       79396    79498     +102     
  Branches    19358    19377      +19     
==========================================
+ Hits        55717    55839     +122     
+ Misses      17896    17877      -19     
+ Partials     5783     5782       -1     

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