[AArch64][SVE] Fix PCS for functions taking/returning scalable types.

The default calling convention needs to save/restore the SVE callee
saves according to the SVE PCS when the function takes or returns
scalable types, even when the `aarch64_sve_vector_pcs` CC is not
specified for the function.

Reviewers: efriedma, paulwalker-arm, david-arm, rengolin

Reviewed By: paulwalker-arm

Differential Revision: https://reviews.llvm.org/D84041