Expand MLIR backend for Ascend NPU support and improve CI/CD

[lucarlig]

MLIR Backend Enhancements

• Expanded MLIR code generation with enhanced type conversion and expression translation
• Improved MLIR builder for better function and statement handling
• Refactored AscendNPU-IR dialect bindings (HIVM, annotation, symbol)

Build & CI/CD Improvements

• Added script to automatically fix TableGen include paths across different build environments
• Optimized GitHub Actions workflow: improved caching strategy, disk space management, and automatic dialect path updates
• Better build artifact cleanup to prevent CI disk space issues

Documentation

• Comprehensive README overhaul: detailed Ascend NPU support explanation, AscendNPU-IR architecture, MLIR backend roadmap
• Improved setup instructions with submodule cloning and troubleshooting guide
• Removed outdated planning doc (content integrated into README)

Testing & Examples

• Reorganized examples into structured directory layout
• Added new test cases with corresponding snapshots
• Updated test infrastructure for MLIR backend validation