Copy decorations when creating new ids. (#1843)

* Copy decorations when creating new ids.

When creating a new value based on an old value, we need to copy the
decorations to the new id.  This change does this in 3 places:

1) The variable holding the return value of the function generated by
merge return should get decorations from the function.

2) The results of the OpPhi instructions should get decorations from the
variable they are replacing in the ssa writer.

3) In local access chain convert the intermediate struct (result of
OpCompositeInsert) generated for the store replacement should get its
decorations from the variable being stored to.

Fixes #1787.