[dajiaohuang]

Fixes #3051.

Changes proposed in this pull request:

• Preserve the receiver and index of compound indexed assignments so expressions with side effects are evaluated once.
• Store the assigned value before invoking an overloaded indexed setter, and preserve the compound assignment expression's result.
• Add regression coverage for StaticArray indexed compound assignment, side effects, stored value, and expression result.

Validation: npm run check passed; npm run build passed; focused compiler regression passed in debug and release, including fixture comparison and instantiation; resolve-elementaccess fixture tests passed after regeneration. The full npm test -- --noColors command reached the asconfig suite but stopped because a nested Windows command could not resolve npm; compiler-suite tests before that stage succeeded. No CI or review result is claimed.

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