spirv: Don't add decorations we ignore to the list
Certain decorations are not used by our code, so don't even bother creating a vtn_decoration and adding them to the list. This is convenient since we don't need to include those (to ignore) in different various switches.
SpvDecorationCounterBuffer and SpvDecorationUserSemantic, which can be safely ignored by the driver, are now ignored only once. This will fix a missing handling of SpvDecorationCounterBuffer decoration (to ignore it).