spirv: Add vtn_mode_uses_ssa_offset()

spirv: Add vtn_mode_uses_ssa_offset()

Just the mode is needed to decide whether SSA offsets are needed, so
make a function that takes that and reuse it for
vtn_pointer_uses_ssa_offset().

This will be used for constant null pointers, that won't have a
vtn_pointer handy.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>