Improve performance in where push the element into non shared Array object

* array.c (ary_ensure_room_for_push): use rb_ary_modify_check() instead of
    rb_ary_modify() to check whether the object can be modified for non shared
    Array object. rb_ary_modify() has the codes for shared Array object too.
    In here, it has condition branch for shared / non shared Array object and
    it can use rb_ary_modify_check() which is smaller function than
    rb_ary_modify() for non shared object.

    rb_ary_modify_check() will be expand as inline function.
    If it will compile with GCC, Array#<< will be faster around 8%.

    [ruby-core:81082] [Bug #13553] [Fix GH-1609]

## Clang 802.0.42
### Before
            Array#<<      9.353M (± 1.7%) i/s -     46.787M in   5.004123s
          Array#push      7.702M (± 1.1%) i/s -     38.577M in   5.009338s
     Array#values_at      6.133M (± 1.9%) i/s -     30.699M in   5.007772s

### After
            Array#<<      9.458M (± 2.0%) i/s -     47.357M in   5.009069s
          Array#push      7.921M (± 1.8%) i/s -     39.665M in   5.009151s
     Array#values_at      6.377M (± 2.3%) i/s -     31.881M in   5.001888s

### Result
Array#<<        -> 1.2% faster
Array#push      -> 2.8% faster
Array#values_at -> 3.9% faster

## GCC 7.1.0
### Before
            Array#<<     10.497M (± 1.1%) i/s -     52.665M in   5.017601s
          Array#push      8.527M (± 1.6%) i/s -     42.777M in   5.018003s
     Array#values_at      7.621M (± 1.7%) i/s -     38.152M in   5.007910s

### After
            Array#<<     11.403M (± 1.3%) i/s -     57.028M in   5.001849s
          Array#push      8.924M (± 1.3%) i/s -     44.609M in   4.999940s
     Array#values_at      8.291M (± 1.4%) i/s -     41.487M in   5.004727s

### Result
Array#<<        -> 8.3% faster
Array#push      -> 4.3% faster
Array#values_at -> 8.7% faster

## Test code
require 'benchmark/ips'

Benchmark.ips do |x|

  x.report "Array#<<" do |i|
    i.times { [1,2] << 3 }
  end

  x.report "Array#push" do |i|
    i.times { [1,2].push(3) }
  end

  x.report "Array#values_at" do |i|
    ary = [1, 2, 3, 4, 5]
    i.times { ary.values_at(0, 2, 4) }
  end

end

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58867 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

array.c

@@ -380,8 +380,11 @@ ary_ensure_room_for_push(VALUE ary, long add_len)
 		}
 	    }
 	}
+	rb_ary_modify(ary);
+    }
+    else {
+	rb_ary_modify_check(ary);
     }
-    rb_ary_modify(ary);
     capa = ARY_CAPA(ary);
     if (new_len > capa) {
 	ary_double_capa(ary, new_len);