[i64-to-i32-lowering] Lower nontrapping float-to-int conversions (#9017)

Add support for `TruncSatSFloat32ToInt64`, `TruncSatUFloat32ToInt64`,
`TruncSatSFloat64ToInt64`, and `TruncSatUFloat64ToInt64` to
`I64ToI32Lowering.cpp`.

In Emscripten mode (`wasm2js --emscripten`), this is effectively a no-op
because `llvm-nontrapping-fptoint-lowering` runs earlier in the pipeline
and rewrites all `TruncSat` operations into trapping conversions before
`i64-to-i32-lowering` executes.

However, landing this is still useful because:
1. Standalone `wasm2js` (without `--emscripten`) does not run
`llvm-nontrapping-fptoint-lowering`, so modules with `i64.trunc_sat_*`
instructions would otherwise fail during JS generation.
2. Direct invocations of `wasm-opt --flatten --i64-to-i32-lowering` will
now correctly eliminate all 64-bit operations on modules using the
nontrapping float-to-int feature.
3. It ensures `I64ToI32Lowering` is self-contained and handles all wasm
i64-producing float truncation opcodes without relying on preceding
lowering passes.
4. Since we removed the use of the `nontrapping-fptoint-lowering` pass
in emscripten itself, I'm hoping we can followup by removing its use
here too and marking it for removal.