fix(compiler): handle null MI in enterIntvAfter

[abmcar]

1. Does this PR affect any open issues?(Y/N) and add issue references (e.g. "fix #123", "re #123".):

• N
• Y

2. What is the scope of this PR (e.g. component or file name):

3. Provide a description of the PR(e.g. more details, effects, motivations or doc link):

• Affects user behaviors
• Contains CI/CD configuration changes
• Contains documentation changes
• Contains experimental features
• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Other

4. Are there any breaking changes?(Y/N) and describe the breaking changes(e.g. more details, motivations or doc link):

• N
• Y

5. Are there test cases for these changes?(Y/N) select and add more details, references or doc links:

• Unit test
• Integration test
• Benchmark (add benchmark stats below)
• Manual test (add detailed scripts or steps below)
• Other

6. Release note

None

[Copilot]

Pull request overview

This PR hardens the compiler’s CGIR split editor by gracefully handling cases where enterIntvAfter is called with a slot index that does not map directly to an instruction, avoiding crashes and correctly placing new interval definitions.

Changes:

• Replaces the assert on a null CgInstruction *MI in CgSplitEditor::enterIntvAfter with logic that:
  • Locates the owning CgBasicBlock for the given CgSlotIndex.
  • Delegates to enterIntvAtEnd when the index is at or past the block end or when no suitable next instruction is found in the same block.
  • Otherwise, finds the next non-null slot index in the same block and inserts the new def before that instruction via defFromParent.
• Preserves existing behavior when a valid MI is found at the index while adding robust handling for “gap” indices that previously triggered the assertion.

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