fix Issue 11906 - Compiler assertion when comparing function pointers

If `f->isImportedSymbol()` is true, the translation from `AddrExp(VarExp(f))` to `SymOffExp(f)` in `AddrExp::optimize` will be skipped.
Therefore, the assertion is just wrong.
Merge pull request #3445 from 9rnsr/fix11906

Issue 11906 - Compiler assertion when comparing function pointers