stop emitting Atomics.fence which is no longer in the SAB spec

Author: kripken

lib/Target/JSBackend/CallHandlers.h

@@ -888,7 +888,9 @@ DEF_BUILTIN_HANDLER(emscripten_int32x4_not, SIMD_Int32x4_not);
 DEF_BUILTIN_HANDLER(emscripten_int32x4_and, SIMD_Int32x4_and);
 DEF_BUILTIN_HANDLER(emscripten_int32x4_or, SIMD_Int32x4_or);
 DEF_BUILTIN_HANDLER(emscripten_int32x4_xor, SIMD_Int32x4_xor);
-DEF_BUILTIN_HANDLER(emscripten_atomic_fence, Atomics_fence);
+DEF_CALL_HANDLER(emscripten_atomic_fence, {
+  return "/* fence */";
+})
 
 DEF_CALL_HANDLER(emscripten_float32x4_select, {
   // FIXME: We really need a more general way of handling boolean types,

lib/Target/JSBackend/JSBackend.cpp

@@ -2662,11 +2662,7 @@ void JSWriter::generateExpression(const User *I, raw_string_ostream& Code) {
     break;
   }
   case Instruction::Fence:
-    if (EnablePthreads) {
-      Code << "Atomics_fence()";
-    } else {
-      Code << "/* fence */"; // no threads, so nothing to do here
-    }
+    Code << "/* fence */"; // not in the spec currently, so nothing to do here
     break;
   }