JIT: fold char loads from movable static readonly strings

[EgorBo]

Fold char loads from movable static readonly strings (e.g. Str = 111111.ToString(); ... str[2]).

Removes the getStringChar special path in fgValueNumberConstLoad in favor of the getObjectContent path, and teaches GetObjectHandleAndOffset to resolve movable static readonly string fields (via getStaticFieldContent(ignoreMovableObjects: false)) so their immutable char content can be folded. The movable handle is only used to read content at compile time, never materialized. Arrays are excluded (mutable contents).

[EgorBo]

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Pull request overview

This PR updates RyuJIT value numbering constant-folding so that char loads from immutable string content can be folded even when the string comes from a movable static readonly reference field, by resolving the field’s runtime value at JIT time and reading immutable object content via getObjectContent.

Changes:

• Extend Compiler::GetObjectHandleAndOffset to recognize addresses based on static readonly ref field loads (via VN patterns) and resolve the field’s object value using getStaticFieldContent(..., ignoreMovableObjects: false) when the target object is immutable (excluding arrays).
• Remove the getStringChar-based special-casing in fgValueNumberConstLoad so folding relies on the general getObjectContent path.