Add SPLIT pass to split 64 bit IR instructions for 32 bit CPUs.

Add generic HIOP instruction for extra backend functionality.
Add support for HIOP to x86 backend.
Use POWI for 64 bit integer x^k, too.
POWI is lowered to a call by SPLIT or the x64 backend.

src/lj_carith.c

@@ -230,6 +230,14 @@ int lj_carith_op(lua_State *L, MMS mm)
 
 /* -- 64 bit integer arithmetic helpers ----------------------------------- */
 
+#if LJ_32
+/* Signed/unsigned 64 bit multiply. */
+int64_t lj_carith_mul64(int64_t a, int64_t b)
+{
+  return a * b;
+}
+#endif
+
 /* Unsigned 64 bit x^k. */
 uint64_t lj_carith_powu64(uint64_t x, uint64_t k)
 {