spirv: Fix structure splitting with per-vertex interface arrays.

spirv: Fix structure splitting with per-vertex interface arrays.

We want to use interface_type, not vtn_var->type.  They're normally
equivalent, but for geometry/tessellation per-vertex interface arrays,
we need to unwrap a level.

Otherwise, we tried to iterate a structure members but instead used
an array length.  If the array length was longer than the number of
fields in the structure, we'd crash.

Fixes the CreatePipelineGeometryInputBlockPositive layer validation
test.

v2: Just use glsl_without_array() on the vtn_var type
    (requested by Jason Ekstrand).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Forbes <chrisforbes@google.com>