Validate pops while ignoring nameless blocks?

[kripken]

@aheejin The fuzzer found another issue on pops in blocks (see test/lit/passes/gto-removals.wast in this PR), so I started to think that we might want a more general solution here. This draft PR changes validation so that nameless blocks, which vanish in the binary format, are ignored for validation. So makeSequence() etc. will never break pop validation.

This seems to work on the test suite. Do you remember if we considered this before? Is there a problem I am missing?