FIR IDE: don't add return for when expressions when converting to block

There doesn't seem to be a good reason why FE1.0 does this. Hence we
won't do it in FIR.

idea/testData/intentions/convertToBlockBody/ifWhenUnit.kt.after.fir

@@ -0,0 +1,14 @@
+enum class AccessMode { READ, WRITE, EXECUTE }
+
+fun foo() {}
+
+fun whenExpr(access: AccessMode, arg: Boolean) {
+    if (arg) {} else {
+        foo()
+        when (access) {
+            AccessMode.READ -> {}
+            AccessMode.WRITE -> {}
+            AccessMode.EXECUTE -> {}
+        }
+    }
+}

idea/testData/intentions/convertToBlockBody/whenUnit.kt.after.fir

@@ -0,0 +1,9 @@
+enum class AccessMode { READ, WRITE, EXECUTE }
+
+fun whenExpr(access: AccessMode) {
+    when (access) {
+        AccessMode.READ -> {}
+        AccessMode.WRITE -> {}
+        AccessMode.EXECUTE -> {}
+    }
+}