rb_ary_behead: do not goto into a branch

I'm not necessarily against every goto in general, but jumping into a
branch is definitely a bad idea.  Better refactor.

array.c

@@ -1524,36 +1524,50 @@ rb_ary_shift_m(int argc, VALUE *argv, VALUE ary)
     return result;
 }
 
-MJIT_FUNC_EXPORTED VALUE
-rb_ary_behead(VALUE ary, long n)
+static VALUE
+behead_shared(VALUE ary, long n)
 {
-    if (n<=0) return ary;
-
+    assert(ARY_SHARED_P(ary));
     rb_ary_modify_check(ary);
-    if (ARY_SHARED_P(ary)) {
     if (ARY_SHARED_ROOT_OCCUPIED(ARY_SHARED_ROOT(ary))) {
-	  setup_occupied_shared:
         ary_mem_clear(ary, 0, n);
     }
     ARY_INCREASE_PTR(ary, n);
+    ARY_INCREASE_LEN(ary, -n);
+    ary_verify(ary);
+    return ary;
 }
-    else {
-	if (RARRAY_LEN(ary) < ARY_DEFAULT_SIZE) {
+
+static VALUE
+behead_transient(VALUE ary, long n)
+{
+    rb_ary_modify_check(ary);
     RARRAY_PTR_USE_TRANSIENT(ary, ptr, {
         MEMMOVE(ptr, ptr+n, VALUE, RARRAY_LEN(ary)-n);
     }); /* WB: no new reference */
-	}
-	else {
-	    ary_make_shared(ary);
-	    goto setup_occupied_shared;
-	}
-    }
     ARY_INCREASE_LEN(ary, -n);
-
     ary_verify(ary);
     return ary;
 }
 
+MJIT_FUNC_EXPORTED VALUE
+rb_ary_behead(VALUE ary, long n)
+{
+    if (n <= 0) {
+        return ary;
+    }
+    else if (ARY_SHARED_P(ary)) {
+        return behead_shared(ary, n);
+    }
+    else if (RARRAY_LEN(ary) >= ARY_DEFAULT_SIZE) {
+        ary_make_shared(ary);
+        return behead_shared(ary, n);
+    }
+    else {
+        return behead_transient(ary, n);
+    }
+}
+
 static VALUE
 ary_ensure_room_for_unshift(VALUE ary, int argc)
 {