Add a cleaner to transform a single conditional as last statement as a guard

[jecisc]

For example, would rewrite

setUp
	super setUp.

	package := RPackageOrganizer default createPackageNamed: 'Package-Test-For-Chanel'.
	extensionPackage := RPackageOrganizer default createPackageNamed: 'ExtensionPackage-Test-For-Chanel'.

	"We only save the formatter if it is not the one used in the tests.
	The reason is that, while debugging a test, if it fails and is rerun, the set up is runned twice and the formatter used in tests will be saved in `previousFormatter`.
	With this guard, we avoid this case."
	RBProgramNode formatterClass = RBSimpleFormatter
		ifFalse: [ previousFormater := RBProgramNode formatterClass.
			RBProgramNode formatterClass: RBSimpleFormatter ]

as

setUp
	super setUp.

	package := RPackageOrganizer default createPackageNamed: 'Package-Test-For-Chanel'.
	extensionPackage := RPackageOrganizer default createPackageNamed: 'ExtensionPackage-Test-For-Chanel'.

	"We only save the formatter if it is not the one used in the tests.
	The reason is that, while debugging a test, if it fails and is rerun, the set up is runned twice and the formatter used in tests will be saved in `previousFormatter`.
	With this guard, we avoid this case."
	RBProgramNode formatterClass = RBSimpleFormatter ifTrue: [ ^ self ].
	previousFormater := RBProgramNode formatterClass.
	RBProgramNode formatterClass: RBSimpleFormatter