[turbofan] Support for %_DoubleHi, %_DoubleLo and %_ConstructDouble.

src/base/bits.h

Index: src/base/bits.h
diff --git a/src/base/bits.h b/src/base/bits.h
index 5c7cd74c610d5f8f59b21214d888d2b284825935..36dcf29a4b284d81d32c3fc5e1dae40fdcfad054 100644
--- a/src/base/bits.h
+++ b/src/base/bits.h
@@ -238,6 +238,11 @@ inline uint32_t UnsignedMod32(uint32_t lhs, uint32_t rhs) {
   return rhs ? lhs % rhs : 0u;
 }
 
+
+// InsertElement64(val, elm, off) returns |val| with the 32 bits at |off|
+// replaced by |elm|. |off| must be either 0 or 1.
+uint64_t InsertElement64(uint64_t val, uint32_t elm, uint8_t off);
+
 }  // namespace bits
 }  // namespace base
 }  // namespace v8