adding a succ to prepare optimize Int64.succ

Author: bobzhang

jscomp/runtime/caml_int64.ml

@@ -72,6 +72,12 @@ let neg_one = mk ~lo:(-1n) ~hi:(-1n)
 
 let neg_signed x =  (x  & 0x8000_0000n) <> 0n
 
+let succ_aux ~x_lo ~x_hi = 
+  let lo =  ( x_lo +~ 1n) &  0xffff_ffffn in  
+  mk ~lo ~hi:(( x_hi +~ if lo = 0n then 1n else 0n) &  0xffff_ffffn)
+let succ (Int64 {lo = x_lo; hi = x_hi} : t) =
+  succ_aux ~x_lo ~x_hi
+  
 let neg (Int64 {lo;hi} ) =
   let other_lo = (lognot lo +~  1n) & 0xffff_ffffn in   
   mk ~lo:other_lo