Inline variable: fix it works correctly for 'when' subject variable

#KT-29870 Fixed

idea/testData/inspectionsLocal/unnecessaryVariable/whenSubject.kt

@@ -0,0 +1,7 @@
+fun test(value: Any): String {
+    return when (val <caret>v = value) {
+        is String -> "$v is String"
+        is Int -> "$v is Int"
+        else -> "$v"
+    }
+}

idea/testData/inspectionsLocal/unnecessaryVariable/whenSubject.kt.after

@@ -0,0 +1,7 @@
+fun test(value: Any): String {
+    return when (value) {
+        is String -> "$value is String"
+        is Int -> "$value is Int"
+        else -> "$value"
+    }
+}