RLSRA: strengthen invariants of (collected) register live intervals

This strengthen invariant checking before proceeding to actual
allocation, namely:

 * checks that each virtual register is at least once defined
   (assigned) and
 * checks that each virtual register is used only after it is defined
   or not used at all.

src/Tinyrossa/TRRegisterLiveInterval.class.st

@@ -99,6 +99,18 @@ TRRegisterLiveInterval >> firstDef [
 	^ nil
 ]
 
+{ #category : #accessing }
+TRRegisterLiveInterval >> firstUse [
+	"Return the first use position for this interval."
+
+	uses do: [:encodedPosition |
+		(self encodesUsePosition: encodedPosition) ifTrue: [
+			^ self decodePosition: encodedPosition
+		].
+	].
+	^ nil
+]
+
 { #category : #initialization }
 TRRegisterLiveInterval >> initializeWithRegister: aTRVirtualRegister [
 	self assert: aTRVirtualRegister isTRVirtualRegister.

src/Tinyrossa/TRReverseLinearScanRegisterAllocator.class.st

@@ -142,7 +142,9 @@ TRReverseLinearScanRegisterAllocator >> allocateRegisters [
 		].
 	].
 	intervals do: [:interval | 
-		self assert: interval start < interval stop.
+		self assert: interval start <= interval stop.
+		self assert: interval firstDef notNil description: 'virtual register not defined (assigned)'.
+		self assert:(interval firstUse isNil or:[interval firstDef < interval firstUse]) description: 'virtual register used before defined (assigned)'.
 	].
 
 	"Create todo (work) list. The list is sorted by interval's end position (#stop).