[mauri870]

Follow-up to #2.

#2 started replacing every constant start or limit with its value, so that the checks parsing Trip.Start/Trip.Limit as text would also see named constants. The emitted file is compiled for every build, though, and a named constant can have a different value in another one. With const width = 32 for amd64 and 64 for arm64, loopvec -split on amd64 hard-coded 32 into the vector loop of for i := 0; i < width; i++, while the scalar fallback kept width. The arm64 build would then vectorize only half the elements. plusOne and i > low loops had always done the same for <= limits and loops that count down.

What changes:

• Constant names are kept. Trip carries the values of a constant start and limit. The checks read those values instead of parsing text: the negative-index check, the short-loop rule and the copy check. A constant is written as its value only when it's built from literals alone (16, 1 - 1). A constant that uses a name keeps the name. i <= width-1 comes out as width.
• Dead code removed. negativeSource could no longer return true after internal/loopir: reject a constant offset that puts an index below zero #2.
• The negative-index check can't overflow. It negates the offset instead of adding it to the bound. It also runs in the same pass over the loop body as the stored-offset checks.
• Bounds pushed past the largest int are rejected. A positive offset that pushes a constant start or limit past the largest int is rejected as that bound. for i := 0; i < math.MaxInt64; i++ { dst[i] = a[i+1] } used to produce an overlap window of math.MaxInt64 + 1, which doesn't compile.
• Tests:
  • offset_negative_constant.txt now checks the rewritten source and runs the loops under both builds.
  • The rejected loops moved to offset_negative_constant_no_rewrite.txt, which adds a loop counting down to 0, a range over a slice, and the two loops pushed past the largest int.
  • bound_arch_constant.txt covers the GOARCH case for <, <=, counting down and >.
• AGENTS.md documents the new rejections and the rule for keeping names.

Tested locally:

• bound_arch_constant.txt fails against internal/loopir: reject a constant offset that puts an index below zero #2's internal/loopir, which writes _ = dst[32-1] where the test expects _ = dst[width-1].
• make test and make test-qemu-arm64 pass.
• make test-update and make fix leave no diff, and make lint reports 0 issues.
• loopvec -json gives identical output before and after over std (1663 loops) and gonum (3252 loops).