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
Reimplement the whole workflow to make that: precise'ness of struct
members won't spread to other non-precise members of the same struct
instance.
Approach:
1. Build the map from symbols to their defining nodes. And for each
object node (StructIndex, DirectIndex, Symbol nodes, etc), generates an
accesschain path. Different AST nodes that indicating a same object
should have the same accesschain path.
2. Along the building phase in step 1, collect the initial set of
'precise' (AST qualifier: 'noContraction') objects' accesschain paths.
3. Start with the initial set of 'precise' accesschain paths, use it as
a worklist, do as the following steps until the worklist is empty:
1) Pop an accesschain path from worklist.
2) Get the symbol part from the accesschain path.
3) Find the defining nodes of that symbol.
4) For each defining node, check whether it is defining a 'precise'
object, or its assignee has nested 'precise' object. Get the
incremental path from assignee to its nested 'precise' object (if
any).
5) Traverse the right side of the defining node, obtain the
accesschain paths of the corresponding involved 'precise' objects.
Update the worklist with those new objects' accesschain paths.
Label involved operations with 'noContraction'.
In each step, whenever we find the parent object of an nested object is
'precise' (has 'noContraction' qualifier), we let the nested object
inherit the 'precise'ness from its parent object.
