[AArch64][NFC]Refactor 'isBitfieldPositioningOp' so that DAG nodes with different...

[AArch64][NFC]Refactor 'isBitfieldPositioningOp' so that DAG nodes with different Opcode are handled with separate helper functions.

Using different helper functions for DAG nodes with different Opcode allows specialization.

- 'isBitfieldExtractOp' [1] shows how specialization based on Opcode could catch more patterns.
- The refactor paves the way (e.g., makes diff clearer) for enhancement in {D135844,D135850,D135852}

[1] https://github.com/llvm/llvm-project/blob/cbd8464595220b5ea76c70ac9965d84970c4b712/llvm/lib/Target/AArch64/AArch64ISelDAGToDAG.cpp#L2163-L2202

Differential Revision: https://reviews.llvm.org/D135843