[mlir][ArmSME] Add option to only enable streaming mode for scalable code (#94759)

This adds a new option
`-enable-arm-streaming=if-contains-scalable-vectors`, which only applies
the selected streaming/ZA modes if the function contains scalable vector
types.

As a NFC this patch also removes the `only-` prefix from the
`if-required-by-ops` mode.