Investigate and validate build_result code paths

[bushidocodes]

A recent bug turned out to be due to calls to build_result being executed with the builder being in the right place. A fix for this was to explicitly call b.position_at_end(self.bb); at the top of the function. This task is to investigate the different code paths where build_result is called to determine if there are either:

1. related logical bugs that the wasm test suite is not picking up
2. a cleaner refactor that doesn't require repositioning the builder