[Draft] Fix for 125246

[dhartglassMSFT]

Random-inline jit stress hits assert(varDsc->lvIsMultiRegRet); in lower.

Global morph replaces a multi-reg-ret return node with a local that doesn't have the bit marked, fix is to set that bit when Assertion copyprop does copy prop.

Also changed the text in some of the assertionprop logging to make the log messages unique.

fixes #125246

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[Copilot]

Pull request overview

Fixes a CoreCLR JIT stress assertion on arm64 where a multi-reg struct return local can be substituted via assertion-driven copy propagation without carrying the required lvIsMultiRegRet marking, causing Lowering::ContainCheckRet to assert.

Changes:

• Make optCopyAssertionProp propagate lvIsMultiRegRet from the original local to the substituted local.
• Make several assertion-prop debug/verbose log headers unique to simplify log analysis.

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