Name of memory export gets mangled in threaded Wasm

[RReverser]

Describe the Bug

WebAssembly memory is normally exported under memory name, but when wasm-bindgen runs on a Wasm file built with an atomics feature, it gets mangled to __wbindgen_export_0.

Steps to Reproduce

1. Build an example in https://github.com/rustwasm/wasm-bindgen/tree/master/examples/raytrace-parallel.
2. Inspect the generated Wasm with wasm2wat or wasm-dis or wasm-objdump etc.
3. Find the memory export.

Expected Behavior

The memory is exported under name "memory" as it does by default in any Wasm builds by LLVM.

Actual Behavior

The memory is exported under name "__wbindgen_export_0" which is not stable enough name to rely on.

Additional Context

From #2102 (comment) via @alexcrichton:

That sounds like a bug in one of the possible passes along the way, likely something that's de-exporting it and then re-exporting it later, forgetting the original export name along the way.