[IR] Fix dangling meta_if matching

[chhzh123]

Description

This PR fixes the dangling meta_if matching issue. Previously, each meta_if needs to be accompanied with a meta_else. Otherwise, different levels of meta_if/else will mix together causing problems.

Examples

For example, in the following case, according to the Python convention, the second meta_else should match the first meta_if based on indention.

with allo.meta_if(...):
    with allo.meta_if(...):
        ...
with allo.meta_else():
    ...

Checklist

• PR's title starts with a category (e.g. [Bugfix], [IR], [Builder], etc)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage (It would be better to provide ~2 different test cases to test the robustness of your code)
• Code is well-documented