Always use x86 inc/dec for add/sub with 1 (#2211)

use inc/dec in fast mode

Author: TheNumbat

backend/amd64/emit.mlp

@@ -1485,9 +1485,9 @@ let emit_instr fallthrough i =
       instr_for_intop op (arg i 1) (res i 0)
   | Lop(Iintop_imm(Iadd, n)) when i.arg.(0).loc <> i.res.(0).loc ->
       I.lea (mem64 NONE n (arg64 i 0)) (res i 0)
-  | Lop(Iintop_imm(Iadd, 1) | Iintop_imm(Isub, -1)) when not !fastcode_flag ->
+  | Lop(Iintop_imm(Iadd, 1) | Iintop_imm(Isub, -1)) ->
       I.inc (res i 0)
-  | Lop(Iintop_imm(Iadd, -1) | Iintop_imm(Isub, 1)) when not !fastcode_flag ->
+  | Lop(Iintop_imm(Iadd, -1) | Iintop_imm(Isub, 1)) ->
       I.dec (res i 0)
   | Lop(Iintop_imm(op, n)) ->
       (* We have i.arg.(0) = i.res.(0) *)