Add IRCONV_NONE for pass-through INT to I64/U64 type change.

src/lj_asm_mips.h

@@ -751,7 +751,7 @@ static void asm_conv(ASMState *as, IRIns *ir)
 	  }
 	}
       } else {
-	if (st64) {
+	if (st64 && !(ir->op2 & IRCONV_NONE)) {
 	  /* This is either a 32 bit reg/reg mov which zeroes the hiword
 	  ** or a load of the loword from a 64 bit address.
 	  */