Generalizes the IsReadOnlyVariable() method, and related methods, so
that they can be used to ask whether pointer result ids are read-only.
Fixes#3324.
When looking at the uses of the result of an instruction, code sinking
assumes that all uses are in a basic block. However, this is not true
if there is a decoration or name for the result of that insturction.
This commit checks for this.
Fixes https://crbug.com/923243.
