Use bit_cast for Runtime_Double{Hi,Lo}.

src/runtime/runtime-maths.cc

Index: src/runtime/runtime-maths.cc
diff --git a/src/runtime/runtime-maths.cc b/src/runtime/runtime-maths.cc
index 68dfa49af839abb99a6027e021a400e83f77f674..24b794cf21782e89c775d08abfdca6caeaa58137 100644
--- a/src/runtime/runtime-maths.cc
+++ b/src/runtime/runtime-maths.cc
@@ -34,9 +34,10 @@ RUNTIME_FUNCTION(Runtime_DoubleHi) {
   HandleScope scope(isolate);
   DCHECK(args.length() == 1);
   CONVERT_DOUBLE_ARG_CHECKED(x, 0);
-  uint64_t integer = double_to_uint64(x);
-  integer = (integer >> 32) & 0xFFFFFFFFu;
-  return *isolate->factory()->NewNumber(static_cast<int32_t>(integer));
+  uint64_t unsigned64 = double_to_uint64(x);
+  uint32_t unsigned32 = static_cast<uint32_t>(unsigned64 >> 32);
+  int32_t signed32 = bit_cast<int32_t, uint32_t>(unsigned32);
+  return *isolate->factory()->NewNumber(signed32);
 }
 
 
@@ -44,8 +45,10 @@ RUNTIME_FUNCTION(Runtime_DoubleLo) {
   HandleScope scope(isolate);
   DCHECK(args.length() == 1);
   CONVERT_DOUBLE_ARG_CHECKED(x, 0);
-  return *isolate->factory()->NewNumber(
-      static_cast<int32_t>(double_to_uint64(x) & 0xFFFFFFFFu));
+  uint64_t unsigned64 = double_to_uint64(x);
+  uint32_t unsigned32 = static_cast<uint32_t>(unsigned64);
+  int32_t signed32 = bit_cast<int32_t, uint32_t>(unsigned32);
+  return *isolate->factory()->NewNumber(signed32);
 }