Using UnionExp in constfold.c
Merge pull request #4149 from WalterBright/unionexp2

[refactor] Using UnionExp in constfold.c
[Refactoring] Reduce indentation
simplify
+jump mode
+interpretFourPointerRelation
don't return NULL if AndAndExp|OrOrExp is void
don't return NULL from ImportStatement
fix Issue 5676