Introduce "Redundant '?: return null'" inspection

#KT-34819 Fixed

idea/testData/inspectionsLocal/redundantElvisReturnNull/.inspection

@@ -0,0 +1 @@
+org.jetbrains.kotlin.idea.inspections.RedundantElvisReturnNullInspection

idea/testData/inspectionsLocal/redundantElvisReturnNull/basic.kt

@@ -0,0 +1,5 @@
+fun foo(): Int? = null
+
+fun test() : Int? {
+    return foo() <caret>?: return null
+}

idea/testData/inspectionsLocal/redundantElvisReturnNull/basic.kt.after

@@ -0,0 +1,5 @@
+fun foo(): Int? = null
+
+fun test() : Int? {
+    return foo()
+}

idea/testData/inspectionsLocal/redundantElvisReturnNull/notInReturn.kt

@@ -0,0 +1,7 @@
+// PROBLEM: none
+fun foo(): Int? = null
+
+fun test() : Int? {
+    val i = foo() <caret>?: return null
+    return 0
+}

idea/testData/inspectionsLocal/redundantElvisReturnNull/notReturnNull.kt

@@ -0,0 +1,6 @@
+// PROBLEM: none
+fun foo(): Int? = null
+
+fun test() : Int? {
+    return foo() <caret>?: return 1
+}

idea/testData/inspectionsLocal/redundantElvisReturnNull/uselessElvis.kt

@@ -0,0 +1,6 @@
+// PROBLEM: none
+fun bar(): Int = 1
+
+fun test() : Int? {
+    return bar() <caret>?: return null // USELESS_ELVIS
+}