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
Full vector and fully specified vector swizzle stores are not affected by this change, only partial swizzles ie swizzles with fewer components than the vector being stored to.
Previously the vector being stored to loaded and any components not specified in the swizzle were used to create a full store to the vector.
While this change generates more SPIR-V instructions, it is necessary for correctness.
Fixes#2518.
