Yul: implement continue, break and return in for loop

[axic]

Add "break" and "continue" with the usual semantics to Yul. This needs some changes (and further tests) in the following optimizer steps (might not be exhaustive):

Trivial:

• ASTCopier (et al.)
• ASTWalker (et al.)
• DataFlowAnalyzer
• RedundantAssignEliminator

The structural simplifier could need some additional rules (remove dead code after unconditional "continue" or "break"), but these might be added later.

[chriseth]

"return" should not take any arguments.

We have a collision between the concept of returning from a function and the return opcode.

Brainstorming for renaming one of them:

• finish
• break_function
• leave
• exit

[chriseth]

Split out the dead code removal into its own issue: #6328

[Marenz]

Couldn't we just reuse break?