IfNullToElvisIntention - better behavior for var's + slightly refactored utility functions

idea/testData/intentions/ifNullToElvis/Var.kt

@@ -0,0 +1,5 @@
+fun foo(p: List<String?>, b: Boolean) {
+    var v = p[0]
+    <caret>if (v == null) return
+    if (b) v = null
+}

idea/testData/intentions/ifNullToElvis/Var.kt.after

@@ -0,0 +1,4 @@
+fun foo(p: List<String?>, b: Boolean) {
+    var v: String? = p[0] ?: <caret>return
+    if (b) v = null
+}