Let empty arrays being flattened not allocate additional state

[methodmissing]

Do not alloc the following for empty Arrays being flattened:

• Stack array
• Memo number table

#array_flatten.yml
prelude: |
  ary = []

benchmark:
  flatten!: ary.flatten!
  flatten: ary.flatten
loop_count: 1000000

lourens@CarbonX1:~/src/ruby/ruby$ /usr/local/bin/ruby --disable=gems -rrubygems -I./benchmark/lib ./benchmark/benchmark-driver/exe/benchmark-driver             --executables="compare-ruby::~/src/ruby/trunk/ruby --disable=gems -I.ext/common --disable-gem"             --executables="built-ruby::./miniruby -I./lib -I. -I.ext/common  -r./prelude --disable-gem" -v --repeat-count=24 $HOME/src/array_flatten.yml
compare-ruby: ruby 2.7.0dev (2019-09-22T17:21:54Z master 142efba93e) [x86_64-linux]
built-ruby: ruby 2.7.0dev (2019-09-22T18:10:18Z opt-flatten-empty-.. ae17889e1e) [x86_64-linux]
Calculating -------------------------------------
                     compare-ruby  built-ruby 
            flatten!       6.234M     31.453M i/s -      1.000M times in 0.160418s 0.031794s
             flatten       6.271M     31.324M i/s -      1.000M times in 0.159468s 0.031924s

Comparison:
                         flatten!
          built-ruby:  31452589.1 i/s 
        compare-ruby:   6233726.0 i/s - 5.05x  slower

                          flatten
          built-ruby:  31324068.7 i/s 
        compare-ruby:   6270832.4 i/s - 5.00x  slower

[nobu]

Doesn't it let Array#flatten! make an unused empty array?