[EH][GC] Fix nested pop after removing ref.cast (#4407)

`ref.cast` can be statically removed when the ref's type is a subtype of
the intended RTT type and either of `--ignore-implicit-traps` or
`--traps-never-happen` is given: https://github.com/WebAssembly/binaryen/blob/083ab9842ec3d4ca278c95e1a33112ae7cd4d9e5/src/passes/OptimizeInstructions.cpp#L1603-L1624

Some more context: https://github.com/WebAssembly/binaryen/pull/4097#discussion_r694456784

But this can create a block in which a `pop` is nested, which makes the
`catch` invalid. The test in this PR is the same as the example given by
@kripken in #4237. This calls the fixup function
`EHUtils::handleBlockNestedPops` at the end of the pass to fix this.

Also, because this pass creates a lot of blocks in other patterns, I
think it is possible there can be other patterns to cause this kind of
`pop` nesting.

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