codegen_ssa: multiply scalable vec size by vscale

[davidtwco]

Part of rust-lang/stdarch#2160. When emitting a memcpy for a scalable vector, the size computed by rustc (num_vectors * element_count * element_ty), since #157915, needs to be multiplied by vscale.

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[rustbot]

rustc_codegen_gcc is developed in its own repository. If possible, consider making this change to rust-lang/rustc_codegen_gcc instead.

cc @antoyo, @GuillaumeGomez

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