Recognize table initialization code in start function and be able to devirtualize based on it

Producers of wasm code prefer small binary size and may use table.fill instructions to initialize parts of a table (or other constructs, like a loop of increasing indices to initialize i31ref table) instead of emitting large element segments.

It would be nice if binaryen offered a flag (e.g. --trust-table-init-code or something like that) that would make it recognize this pattern and treat it as iff the regions were initialized in element sections and therefore be immutable and allow devirtualization / loads of the table to be folded away.

It could be an opt-in flag.

(In some sense an export of a table - e.g. in multi module scenarios - is in similar situation. One may want with a wasm-merge+wasm-opt+wasm-split the ability to devirtualize -- and would have an assumption that the exported table isn't used by 3rd party modules)

Start by tracing how Binaryen handles table.fill, start-function initialization code, and element segments, then compare that with the existing devirtualization and table-load folding behavior. Define the opt-in flag semantics and verify that recognized initialized regions are treated like immutable element-section regions without changing unrecognized tables.