Create singleton sets in LiveVariableAnalyzer where possible

This will be slightly more efficient than the existing approach.

Signed-off-by: Graham <gpe@openrs2.dev>

deob/src/main/java/dev/openrs2/deob/analysis/LiveVariableAnalyzer.kt

@@ -47,6 +47,8 @@ public class LiveVariableAnalyzer(owner: String, method: MethodNode) :
     private fun IntSet.plus(element: Int): IntSet {
         if (contains(element)) {
             return this
+        } else if (isEmpty()) {
+            return IntSets.singleton(element)
         }
 
         val newSet = IntOpenHashSet(this)