[mlir][linalg] Set explicit insertion point in pad_tensor patterns.

Insert ops replacing pad_tensor in front of the associated tansfer_write / insert_slice op. Otherwise we may end up with invalid ir if one of the remaining tansfer_write / insert_slice operands is defined after the pad_tensor op.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D106162