I64ToI32Lowering: Document non-trapping float truncation behavior. NFC (#9022)

Document that `I64ToI32Lowering` is designed for the `wasm2js` pipeline
and does not preserve strict WebAssembly trapping semantics for
float-to-int conversions.

Also add comments in `lowerTruncFloatToInt` noting that the emitted
32-bit truncations are translated to JavaScript bitwise operations in
`wasm2js` and thus do not trap on out-of-range values, NaN, or infinity.

See: #9017