Preserve checked overflow for unsigned range additions

[Copilot]

Tier 1 assertion propagation could incorrectly remove an unsigned addition’s overflow check because JIT ranges use signed intervals.

• Range analysis

  • Reject unsigned ranges that straddle signed zero.
  • Require sums to remain representable by the signed range model.

• Regression coverage

  • Cover the reported checked-addition failure and sign-straddling ranges.

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[Copilot]

Pull request overview

This PR adjusts CoreCLR JIT range arithmetic to avoid incorrectly proving that certain unsigned checked additions cannot overflow when the underlying range model is expressed as signed intervals, and adds a regression test to lock the behavior in.

Changes:

• Make RangeOps::Add(..., unsignedAdd: true) bail out to Unknown when either operand’s signed constant range straddles zero (non-monotonic under unsigned interpretation).
• Prevent forming constant range sums for unsigned adds unless the endpoint addition is also signed-non-overflowing, avoiding unsafe signed overflow during range endpoint computation.
• Add a JIT regression test covering a checked-unsigned-add overflow and a sign-straddling range scenario, and include it in the regression csproj.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/coreclr/jit/rangecheck.h	Tightens unsigned range-add handling to avoid unsound overflow-check removal when ranges straddle signed zero or when signed endpoint sums would overflow.
src/tests/JIT/Regression/JitBlue/Runtime_130431/Runtime_130431.cs	Adds regression coverage ensuring checked unsigned additions still throw OverflowException in the affected scenarios.
src/tests/JIT/Regression/Regression_ro_2.csproj	Includes the new regression test source in the build.