It is invalid if the same decoration is applied to the same id multiple
times. This adds a check before adding a decoration that the decoration
is not already in the list. If it is, then the duplicate is not added.
Fixes#3627
spirv_decorate_id will generate OpDecorateId. The parameter list should
accept variables as part of decorations. This is because OpDecorateId
allows this. The spec says:
All such <id> Extra Operands must be constant instructions or
OpVariable instructions.