Make IrExpression.type nullable.

compiler/testData/ir/irText/callWithReorderedArguments.txt

@@ -17,16 +17,16 @@ IrFile /callWithReorderedArguments.kt
   IrFunction public fun test(): kotlin.Unit
     IrExpressionBody
       BLOCK type=kotlin.Unit
-        CALL .foo
-          a: CALL .noReorder1
-          b: CALL .noReorder2
+        CALL .foo type=kotlin.Unit operator=
+          a: CALL .noReorder1 type=kotlin.Int operator=
+          b: CALL .noReorder2 type=kotlin.Int operator=
         BLOCK type=kotlin.Unit
           LOCAL tmp0
             IrLocalVariable val tmp0: kotlin.Int
-              CALL .reordered1
+              CALL .reordered1 type=kotlin.Int operator=
           LOCAL tmp1
             IrLocalVariable val tmp1: kotlin.Int
-              CALL .reordered2
-          CALL .foo
-            a: GET_VAR tmp1
-            b: GET_VAR tmp0
+              CALL .reordered2 type=kotlin.Int operator=
+          CALL .foo type=kotlin.Unit operator=
+            a: GET_VAR tmp1 type=kotlin.Int
+            b: GET_VAR tmp0 type=kotlin.Int