glsl: stop copying struct and interface member names

We are currently copying the name for each member dereference
but we can just share a single instance of the string provided
by the type.

This change also stops us recalculating the field index
repeatedly.

Reviewed-by: Thomas Helland <thomashelland90@gmail.com>

src/compiler/glsl/ir_clone.cpp

@@ -194,8 +194,10 @@ ir_dereference_array::clone(void *mem_ctx, struct hash_table *ht) const
 ir_dereference_record *
 ir_dereference_record::clone(void *mem_ctx, struct hash_table *ht) const
 {
+   const char *field_name =
+      this->record->type->fields.structure[this->field_idx].name;
    return new(mem_ctx) ir_dereference_record(this->record->clone(mem_ctx, ht),
-					     this->field);
+                                             field_name);
 }
 
 ir_texture *