[mlir][Linalg] Replace SimplePad with PadTensor in hoist-padding

This is the last revision to migrate using SimplePadOp to PadTensorOp, and the
SimplePadOp is removed in the patch. Update a bit in SliceAnalysis because the
PadTensorOp takes a region different from SimplePadOp. This is not covered by
LinalgOp because it is not a structured op.

Also, remove a duplicated comment from cpp file, which is already described in a
header file. And update the pseudo-mlir in the comment.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D95671