Skip nullcheck frozen object in unbox optimization

[MichalStrehovsky]

Fixes the build crash in #121697.

Since with TrimMode=partial, we consider all static fields targets of reflection, we cannot allow inlining their contents. With full trimming allowed, the optimization is unlocked and we hit an assert when trying to nullcheck a constant.

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Pull Request Overview

This PR fixes a build crash (#121697) that occurs when trying to perform null checks on frozen objects during unbox optimization. The issue arises with full trimming enabled, where static field contents can be inlined, and the JIT encounters an assertion failure when attempting to null-check constants (frozen/preinitialized objects).

• Conditionally generates null checks only when fgAddrCouldBeNull returns true
• Skips null checks for frozen/preinitialized objects that cannot be null
• Adds explanatory comment for the optimization

[jakobbotsch]

we hit an assert when trying to nullcheck a constant

What assert?

[MichalStrehovsky]

we hit an assert when trying to nullcheck a constant

What assert?

gtNewNullCheck asserts that the value can be null.

[EgorBo]

Interesting, I can see why we never hit it on CoreCLR before