Case-insensitive regexes shouldn't be replaced with strings in split

> "oneSpLiTtwo".split("split")
=> ["oneSpLiTtwo"]
> "oneSpLiTtwo".split(/split/i)
=> ["one", "two"]

Expected behavior

Don't trigger RedundantSplitRegexpArgument for case-insensitive regexes

Actual behavior

Rubocop triggers RedundantSplitRegexpArgument for case-insensitive regexes

Steps to reproduce the problem

Run rubocop on

content.split(/separator/i)

RuboCop version

-> be rubocop -V
1.10.0 (using Parser 3.0.0.0, rubocop-ast 1.4.1, running on ruby 2.6.5 x86_64-darwin19)
  - rubocop-performance 1.10.0
  - rubocop-rails 2.9.1