Previously we use vectors of objects and move semantics to handle
ownership. That approach has the flaw that inserting an object into
the middle of a vector, which may trigger a vector reallocation,
can invalidate some addresses taken from instructions.
Now the in-memory representation internally uses vector of unique
pointers to handle ownership. Since objects are explicitly heap-
allocated now, pointers to them won't be invalidated by vector
resizing anymore.
Add a pass to freeze spec constants to their default values. This pass does
not fold the frozen spec constants and does not handle SpecConstantOp
instructions and SpecConstantComposite instructions.
