[wasm-split] Slow splitting many modules

[biggs0125]

For a large file, wasm-split with --multi-split takes 3.5+ minutes to separate the file into ~350 submodules.

My understanding is that --multi-split basically invokes split mode iteratively. I imagine it would be faster if it processed the entire main module at once [O(n)] rather than walking the entire (albeit slowly shrinking) main module many times [O(n^2)].

[tlively]

cc @aheejin

[aheejin]

#7956 reduced the splitting time of a Dart test case that splits a module into 350 submodules by around 66%.

[aheejin]

Running time improvements:
After #7956: ~80s
After #8221: ~16s