[release/10.0] JIT: Use r9,r10 for GS cookie check in Swift reverse pinvokes

[jakobbotsch]

Customer Impact

• Customer reported
• Found internally

The JIT corrupts return values from x64 Swift reverse pinvokes when 4 registers are used to return in if the method simultaneously uses unsafe value types that induce a GS cookie check.

Regression

• Yes
• No

Introduced by #119864.

Testing

Internal stress testing (that forces GS cookie check) caught this in our existing Swift tests.

Risk

Low. Change only affects Swift reverse pinvokes with GS cookie checks.

[Copilot]

Pull Request Overview

This PR fixes a regression in the JIT compiler where return values from Swift reverse pinvokes were corrupted when 4 registers were used for return values and the method required a GS (Guard Stack) cookie check. The fix changes the registers used for GS cookie checks in Swift calling convention from R8/R9 to R9/R10, avoiding conflicts with Swift return value registers.

• Updates register allocation for GS cookie checks in Swift calling convention
• Prevents corruption of Swift reverse pinvoke return values when using unsafe code

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