effects: improve nothrow modeling for Core._svec_len

[aviatesk]

Adds a dedicated _svec_len_nothrow function that does more precise :nothrow modeling of Core._svec_len implemented in #59548.

[Copilot]

Pull Request Overview

This PR improves the nothrow modeling for Core._svec_len by introducing a dedicated _svec_len_nothrow function that provides more precise :nothrow analysis. Previously, the nothrow check was based on whether the result was a constant, but now it checks if the argument is a subtype of SimpleVector.

• Adds _svec_len_nothrow function for precise nothrow modeling
• Updates _builtin_nothrow to use the new function instead of constant-based checking
• Adds test to verify length on Core.SimpleVector is now nothrow

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
Compiler/src/tfuncs.jl	Adds _svec_len_nothrow function and updates nothrow logic for Core._svec_len
Compiler/test/effects.jl	Adds test verifying nothrow behavior for length on Core.SimpleVector

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[oscardssmith]

why is this on the backport label? It doesn't seem like a bugfix?

[aviatesk]

Since this is simple enough to be backported, while this is necessary for tools like JET relying on the compiler to behave reasonably on 1.12.