* Introduce `ret_load` ZIR instruction which does return semantics
   based on a corresponding `ret_ptr` instruction. If the return type of
   the function has storage for the return type, it simply returns.
   However if the return type of the function is by-value, it loads the
   return value from the `ret_ptr` allocation and returns that.

 * AstGen: improve `finishThenElseBlock` to not emit break instructions
   after a return instruction in the same block.

 * Sema: `ret_ptr` instruction works correctly in comptime contexts.
   Same with `alloc_mut`.

The test case with a recursive inline function having an implicitly
comptime return value now has a runtime return value because of the fact
that it calls a function in a non-comptime context.