[mlir][linalg] Refactor PadTensorOpVectorizationPattern (NFC)

* Rename PadTensorOpVectorizationPattern to GenericPadTensorOpVectorizationPattern.
* Make GenericPadTensorOpVectorizationPattern a private pattern, to be instantiated via populatePadTensorOpVectorizationPatterns.
* Factor out parts of PadTensorOpVectorizationPattern into helper functions.

This commit prepares PadTensorOpVectorizationPattern for a series of subsequent commits that add more specialized PadTensorOp vectorization patterns.

Differential Revision: https://reviews.llvm.org/D103681