Compiler: add smalltags for IR node types and optimize _useref_getindex (#60831)
## Summary
- Add small tags for IR node types commonly used in `_useref_getindex`:
Argument, GotoIfNot, ReturnNode, EnterNode, PiNode, PhiNode, PhiCNode,
UpsilonNode, GlobalRef, GotoNode, and QuoteNode
- This enables the compiler to generate efficient switch statements
based on the type tags instead of pointer comparisons
- Split `_useref_getindex` into separate `op==1` and `op>1` paths to
avoid redundant comparisons
## Test plan
- [x] Julia builds successfully
- [x] Verified LLVM IR shows efficient switch statements on smalltags
🤖 Generated with [Claude Code](https://claude.com/claude-code)
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Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
Co-authored-by: Jameson Nash <vtjnash@gmail.com>