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This pass finds all string.const and creates globals for them. After this transform, no
string.const appears anywhere but in a global, and each string appears in one global
which is then global.get-ed everywhere.
This avoids overhead in VMs where executing a string.const is an allocation, and is
also a good step towards imported strings. For that, this pass will be extended from
gathering to a full lowering pass, which will first gather into globals as this pass does,
and then turn each of those globals with a string.const into an imported externref.
(For that reason this pass is in a file called StringLowering, as the two passes will
share much of their code, and the larger pass should decide the name I think.)
This pass runs in -O2 and above. Repeated executions have no downside (see
details in code).
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