[WebAssembly] Nullify unnecessary setjmp calls

D107530 did a small optimization that, if a function contains `setjmp`
calls but not other calls that can `longjmp`, we don't do SjLj
transformation on those `setjmp` calls, because they don't have
possibilities of returning from `longjmp`.

But we should remove those `setjmp` calls even in that case, because
Emscripten doesn't provide that function, assuming it is lowered away by
SjLj transformation. `setjmp` always returns 0 when called directly, so
this CL replaces them with `i32 0`.

Fixes https://github.com/emscripten-core/emscripten/issues/15679.

Reviewed By: dschuff

Differential Revision: https://reviews.llvm.org/D116619