fix(vpto): normalize vector shift counts

[Zhendong404]

Summary

• require signed vector shift-count carriers while preserving lhs/result types
• normalize PTODSL and VMI-to-VPTO shift counts with bitcasts
• cover signedness verification and lowering regressions

Validation

• cmake --build /home/zhangzhendong/ptoas-workspace/builds/issue-1220 --target pto-test-opt -j8
• llvm-lit: vmi_to_vpto_shrui, vmi_to_vpto_shli, vmi_to_vpto_shrsi, vshift_mixed_signedness_verify
• ptodsl/tests/test_vector_cube_ops.py
• ptodsl/tests/test_shift_count_normalization.py
• ptodsl/tests/test_vmi_vshr_signedness.py