Fix slice handling in FunctionList, BasicBlockList, and TagList

[3rdit]

The standard Python slicing syntax, i.e. [:], [2:], [:5].. (anything with an open end) fail with TypeError on FunctionList, BasicBlockList and TagList. Other valid slices like [0:len(list)] would also cause an IndexError.

The slice bounds checking compares i.start and i.stop directly against integers, but Python sets these to None for open-ended slices. This means comparing None < 0 raises TypeError. The check i.stop >= len(self) also rejects valid Python where stop equals length.

This PR replaces the manual bounds checking with slice.indices() that will handle None values and normalise the bounds correctly. Other places in the codebase already do this.

[CLAassistant]

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