Add fixup function for nested pops in catch (#4348)

This adds `EHUtils::handleBlockNestedPops`, which can be called at the
end of passes that has a possibility to put `pop`s inside `block`s. This
method assumes there exists a `pop` in a first-descendant line, even
though it can be nested within a block. This allows a `pop` to be nested
within a `block` or a `try`, but not a `loop`, since that means the
`pop` can run multile times. In case of `if`, `pop` can exist only in
its condition; if a `pop` is in its true or false body, that's not in
the first-descendant line.

This can be useful when optimization passes create blocks to do          
transformations. Wrapping expressions wiith a block does not change      
semantics most of the time, but if pops happen to be inside a block      
generated by those passes, they can result in invalid binaries.

To test this, this adds `passes/test_passes.cpp`, which is intended to
contain multiple test passes that test a single (or more) utility
functions separately. Without this kind of pass, it is hard to test
various cases in which nested `pop`s can be generated in existing
passes. This PR also adds `PassRegistry::registerTestPass`, which
registers a pass that's intended only for internal testing and does not
show up in `wasm-opt --help`.

Fixes #4237.

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