JIT: eliminate bounds checks for "i != arr.Length" loops

[AndyAyersMS]

Loops with i != end previously kept their per-iteration bounds checks while the equivalent < / > forms did not: loop cloning bailed on GT_NE, and RangeCheck couldn't refine the IV's range from a "back-edge != end" assertion.

Recognize GT_NE with stride exactly +/-1 as an increasing or decreasing loop in NaturalLoopIterInfo, extend optDeriveLoopCloningConditions to emit the right per-access and zero-trip conditions for GT_NE, and add a new "init RELOP end" cloning condition so that a misordered init (which for GT_NE would wrap the IV through the type rather than exit at the first test) falls back to the slow path. Builds on #129176 which fixes the latent decreasing-loop soundness gap.

Also extend optCreateJTrueBoundsAssertion to create CompareCheckedBound assertions for EQ/NE against a checked bound, and teach RangeCheck::MergeAssertion to tighten ranges using X != bound / X == bound. This catches NE-loop BCE in cases that fall outside loop cloning (e.g. loops with side-effecting calls in the body). For equality assertions where the non-bound side is a constant, defer to the existing LCLVAR-based equality assertion path to preserve downstream consumers that rely on it (e.g. proving "len > 0" after a "len != 0" check).

Codegen for for (int i = 0; i != src.Length; i++) sum += src[i] drops from 64 to 30 bytes.

Fixes #84697.

[Copilot]

Pull request overview

Updates CoreCLR JIT loop analysis, loop cloning, assertion propagation, and range analysis to enable bounds-check elimination for i != end loop forms (with strict stride constraints), aligning them more closely with equivalent < / > loops and improving both optimization and soundness.

Changes:

• Teach NaturalLoopIterInfo to classify GT_NE loop tests as increasing/decreasing only when the IV step is exactly +1/-1.
• Extend loop cloning (optDeriveLoopCloningConditions) to accept GT_NE tests and add the necessary entry/zero-trip and per-access guards to keep the fast clone sound.
• Extend assertion creation + RangeCheck merging so ==/!= assertions against checked bounds can tighten IV ranges and enable BCE even when loop cloning can’t apply.
• Add a JIT regression test covering both the optimization shape and key soundness cases.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/tests/JIT/Regression/JitBlue/Runtime_84697/Runtime_84697.cs	Adds regression coverage for i != Length loops across arrays/strings/spans and includes soundness checks for misordered init and decreasing-loop two-array scenarios.
src/coreclr/jit/flowgraph.cpp	Extends increasing/decreasing loop classification to recognize GT_NE only for stride ±1 to avoid non-terminating/wrapping cases.
src/coreclr/jit/loopcloning.cpp	Enables loop cloning reasoning for GT_NE, adds ordered entry guards and an init-vs-limit guard to prevent wraparound on the fast path, and derives per-access conditions for GT_NE.
src/coreclr/jit/assertionprop.cpp	Allows optCreateJTrueBoundsAssertion to create checked-bound equality assertions (EQ/NE) where useful for RangeCheck while preserving existing constant-equality assertion behavior.
src/coreclr/jit/rangecheck.cpp	Teaches RangeCheck to tighten ranges from X == bound / X != bound assertions where bound is a checked bound, enabling BCE in non-cloneable cases.