Improve TestUseTypeCheckEqualsOnBoolean

Various improvements for `TestUseTypeCheckEqualsOnBoolean`.

* Fix fatal error. Previously, `TestUseTypeCheckEqualsOnBoolean` would abort
  with "PHP Catchable fatal error: Object of class
  PhpParser\Node\Expr\BinaryOp\Concat could not be converted to string" when
  it encountered something like `false == ${'a' . $id}`.
* Add check for left side of the equal.
* Don't treat `$false` and `$true` as boolean literals.

See https://gist.github.com/redbeardcreator/769f04a0b9401125219c for a sample
PHP file.