spirv: Expose stage enum conversion in vtn_private.h

Refactor it to not fail, just return MESA_SHADER_NONE.  Caller
takes care of handling error.  Exposed so can be used by spirv2nir
program.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25461>

src/compiler/spirv/spirv_to_nir.c

@@ -4540,8 +4540,8 @@ vertices_in_from_spv_execution_mode(struct vtn_builder *b,
    }
 }
 
-static gl_shader_stage
-stage_for_execution_model(struct vtn_builder *b, SpvExecutionModel model)
+gl_shader_stage
+vtn_stage_for_execution_model(SpvExecutionModel model)
 {
    switch (model) {
    case SpvExecutionModelVertex:
@@ -4577,8 +4577,7 @@ stage_for_execution_model(struct vtn_builder *b, SpvExecutionModel model)
    case SpvExecutionModelMeshEXT:
       return MESA_SHADER_MESH;
    default:
-      vtn_fail("Unsupported execution model: %s (%u)",
-               spirv_executionmodel_to_string(model), model);
+      return MESA_SHADER_NONE;
    }
 }
 
@@ -4598,8 +4597,12 @@ vtn_handle_entry_point(struct vtn_builder *b, const uint32_t *w,
    unsigned name_words;
    entry_point->name = vtn_string_literal(b, &w[3], count - 3, &name_words);
 
+   gl_shader_stage stage = vtn_stage_for_execution_model(w[1]);  
+   vtn_fail_if(stage == MESA_SHADER_NONE,
+               "Unsupported execution model: %s (%u)",
+               spirv_executionmodel_to_string(w[1]), w[1]);
    if (strcmp(entry_point->name, b->entry_point_name) != 0 ||
-       stage_for_execution_model(b, w[1]) != b->entry_point_stage)
+       stage != b->entry_point_stage)
       return;
 
    vtn_assert(b->entry_point == NULL);

src/compiler/spirv/vtn_private.h

@@ -1071,4 +1071,6 @@ struct vtn_ssa_value *vtn_cooperative_matrix_insert(struct vtn_builder *b, struc
 nir_deref_instr *vtn_create_cmat_temporary(struct vtn_builder *b,
                                            const struct glsl_type *t, const char *name);
 
+gl_shader_stage vtn_stage_for_execution_model(SpvExecutionModel model);
+
 #endif /* _VTN_PRIVATE_H_ */