spirv: Move the emit a 'return value' store logic into own function

spirv: Move the emit a 'return value' store logic into own function

Right now, only the structured CF path emits 'return value' stores when
an SpvOpReturnValue opcode is found. Move the emit 'return value' logic
in a separate function so we can use it from the unstructured path as well.

v2 (Karol): rephrased and removed unstructured changes

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/2401>