[mlir][tensor] Refine the semantics of `createPadHighOp` (#109667)

Refine `createPadHighOp` so that the output tensor is required to be
statically shaped. This is to prevent the current behaviour, which is
incorrect:

>  // If `type` has dynamic dimensions the padding width is set to zero.

The actual padding width should be set to: `%new_dim - %old_dim`, where
%new_dim` and `%old_dim` are defined via e.g. `tensor.dim` Op applied to
output and input tensors, respectively.

This PR is an attempt to clarify the semantics surrounding dynamic
shapes in preparation for adding support for scalable vectors to the
pack/unpack logic in Tensor/Linalg (dynamic shapes is what we use to
model scalable (*) sizes at the Tensor/MemRef level).

(*) Scalable as in Arm's Scalable Vector Extension (SVE)

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