Optimize ReorderLocals maps to vectors (#2727)
Optimize ReorderLocals maps to vectors (#2727)

The original code here is quite old and I guess we assumed
that it was ok to use maps for this, but on really huge amounts
of locals it ends up mattering a lot. This makes the pass 3x
faster on poppler after --flatten --flatten (double flatten creates
lots of new locals, and is generally similar to what wasm2js
does).
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