[Fix #10325] Enhance Style/RedundantCondition for the case that variable assignments in each branch

[nobuyo]

Fixes #10325.

The example using nil? given in the issue is not supported here, as it would destroy the code whose intent is to execute else_branch when value == false.

# not supported
def lock_retrier=(value)
  if value.nil?
    @lock_retrier = NullLockRetrier.new
  else
    @lock_retrier = value
  end
end

# supported
def lock_retrier=(value)
  if value
    @lock_retrier = NullLockRetrier.new
  else
    @lock_retrier = value
  end
end

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[nobuyo]

Should the following patterns be covered as well?
I have a feeling that it may be overkill.

if foo
  test.bar foo
else
  test.bar 'baz'
end
# => test.bar foo || 'baz'

if foo
  test.bar = foo
else
  test.bar = 'baz'
end
# => test.bar = foo || 'baz'

[nobuyo]

@rubocop/rubocop-core I pushed af27f01 with the split as I dared. If you give me your thoughts, I'd like to squash or revert it.

[bbatsov]

The changes look reasonable to me. Thanks!