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_iropt.h

@@ -141,6 +141,12 @@ LJ_FUNC IRType lj_opt_narrow_forl(cTValue *forbase);
 /* Optimization passes. */
 LJ_FUNC void lj_opt_dce(jit_State *J);
 LJ_FUNC int lj_opt_loop(jit_State *J);
+#if LJ_HASFFI && LJ_32
+LJ_FUNC void lj_opt_split(jit_State *J);
+#else
+#define lj_opt_split(J)		UNUSED(J)
+#endif
+
 #endif
 
 #endif