spirv: Hand-roll fewer vtn_ssa_value creations

spirv: Hand-roll fewer vtn_ssa_value creations

Previously, we created our vtn_ssa_value in _vtn_variable_load_store
manually as we did the recursive load/store.  Instead, we now create the
SSA value before calling into the recursive function.  This is a tiny
bit less efficient but it removes a case of hand-rolling vtn_ssa_value
creation.  For symmetry, we make _vtn_block_load_store assume the value
is already created.  Finally, we remove a trivial hand-rolled case in
vtn_composite_extract.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5278>