[flang][Lower] Emit exiting branches from within constructs (#92455)

When lowering IfConstruct, CaseConstruct, and SelectTypeConstruct, emit
branches that exit the construct in each block that is still
unterminated after the FIR has been generated in it.

The same thing may be needed for SelectRankConstruct, once it's
supported.

This eliminates the need for inserting branches in `genFIR(Evaluation)`.

Follow-up to PR https://github.com/llvm/llvm-project/pull/91614.