[flang] Implement semantics for computed GOTO. Note that a PGI extension

will implicitly convert a float to integer (and issue a warning).
This commit does not implement that extension.

Original-commit: flang-compiler/f18@b85755f26fef767850572ef5d3deea87258d87a7
Reviewed-on: https://github.com/flang-compiler/f18/pull/360
Tree-same-pre-rewrite: false