Fix false positives for Migration/SchemaAdditionMethodsNoPost cop

What does this MR do and why?

We allow add_column and create_table methods inside the down method in post-deploy migrations. But if these methods were wrapped inside a statement like if then the cop was raising an error. For example, in the following, the 2nd example was being incorrectly flagged as an offense:

def down
  add_column(:table, :column)
end

def down
  add_column(:table, :column) unless column_exists(:table, :column)
end

How to set up and validate locally

1. Run spec/rubocop/cop/migration/schema_addition_methods_no_post_spec.rb on master and on this branch.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #414251 (closed)