Add pseudo-instruction Ladjust_trap_depth (ocaml#2322)

Ladjust_trap_depth replaces dummy Lpushtrap generated in linearize of
Iexit to notify assembler generation about updates to the
stack. Ladjust_trap_depth is used to keep the virtual stack pointer in
sync and emit dwarf information, without emitting any assembly
instructions. It therefore avoids generating dead code.

This patch is extract from PR1482 @lthls

asmcomp/amd64/emit.mlp

@@ -867,6 +867,11 @@ let emit_instr fallthrough i =
       D.text ()
   | Lentertrap ->
       ()
+  | Ladjust_trap_depth { delta_traps; } ->
+      (* each trap occupies 16 bytes on the stack *)
+      let delta = 16 * delta_traps in
+      cfi_adjust_cfa_offset delta;
+      stack_offset := !stack_offset + delta
   | Lpushtrap { lbl_handler; } ->
       let load_label_addr s arg =
         if !Clflags.pic_code then

asmcomp/arm/emit.mlp

@@ -877,6 +877,11 @@ let emit_instr i =
         end
     | Lentertrap ->
         0
+    | Ladjust_trap_depth { delta_traps } ->
+        (* each trap occupies 8 bytes on the stack *)
+        let delta = 8 * delta_traps in
+        cfi_adjust_cfa_offset delta;
+        stack_offset := !stack_offset + delta; 0
     | Lpushtrap { lbl_handler; } ->
         let s = emit_load_handler_address lbl_handler in
         stack_offset := !stack_offset + 8;

asmcomp/arm64/emit.mlp

@@ -495,6 +495,7 @@ module BR = Branch_relaxation.Make (struct
         + begin match lbl2 with None -> 0 | Some _ -> 1 end
     | Lswitch jumptbl -> 3 + Array.length jumptbl
     | Lentertrap -> 0
+    | Ladjust_trap_depth _ -> 0
     | Lpushtrap _ -> 4
     | Lpoptrap -> 1
     | Lraise k ->
@@ -875,6 +876,11 @@ let emit_instr i =
 *)
     | Lentertrap ->
         ()
+    | Ladjust_trap_depth { delta_traps } ->
+        (* each trap occupies 16 bytes on the stack *)
+        let delta = 16 * delta_traps in
+        cfi_adjust_cfa_offset delta;
+        stack_offset := !stack_offset + delta
     | Lpushtrap { lbl_handler; } ->
         `	adr	{emit_reg reg_tmp1}, {emit_label lbl_handler}\n`;
         stack_offset := !stack_offset + 16;