[NFC] Avoid quadratic time in StackIROptimizer::removeUnneededBlocks() (#6859)

This is in quite ancient code, so it's a long-standing issue, but it got worse
when we enabled StackIR in more situations (#6568), which made it more
noticeable, I think.

For example, testing on test_biggerswitch in Emscripten, the LLVM part
is pretty slow too so the Binaryen slowdown didn't stand out hugely, but
just doing

wasm-opt --optimize-level=2 input.wasm -o output.wasm

(that is, do no work, but set the optimize level to 2 so that StackIR opts
are run) used to take 28 seconds (!). With this PR that goes down to less
than 1.

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