Add an implicit cast to Unit in returns from a constructor.

compiler/testData/ir/irText/expressions/implicitCastInReturnFromConstructor.kt

@@ -0,0 +1,5 @@
+class C() {
+    constructor(x: Any?) : this() {
+        if (x is Unit) return x
+    }
+}

compiler/testData/ir/irText/expressions/implicitCastInReturnFromConstructor.txt

@@ -0,0 +1,31 @@
+FILE fqName:<root> fileName:/implicitCastInReturnFromConstructor.kt
+  CLASS CLASS name:C modality:FINAL visibility:public superTypes:[kotlin.Any]
+    $this: VALUE_PARAMETER INSTANCE_RECEIVER name:<this> type:<root>.C
+    CONSTRUCTOR visibility:public <> () returnType:<root>.C [primary]
+      BLOCK_BODY
+        DELEGATING_CONSTRUCTOR_CALL 'public constructor <init> () [primary] declared in kotlin.Any'
+        INSTANCE_INITIALIZER_CALL classDescriptor='CLASS CLASS name:C modality:FINAL visibility:public superTypes:[kotlin.Any]'
+    CONSTRUCTOR visibility:public <> (x:kotlin.Any?) returnType:<root>.C
+      VALUE_PARAMETER name:x index:0 type:kotlin.Any?
+      BLOCK_BODY
+        DELEGATING_CONSTRUCTOR_CALL 'public constructor <init> () [primary] declared in <root>.C'
+        WHEN type=kotlin.Unit origin=IF
+          BRANCH
+            if: TYPE_OP type=kotlin.Boolean origin=INSTANCEOF typeOperand=kotlin.Unit
+              GET_VAR 'x: kotlin.Any? declared in <root>.C.<init>' type=kotlin.Any? origin=null
+            then: RETURN type=kotlin.Nothing from='public constructor <init> (x: kotlin.Any?) declared in <root>.C'
+              TYPE_OP type=kotlin.Unit origin=IMPLICIT_COERCION_TO_UNIT typeOperand=kotlin.Unit
+                GET_VAR 'x: kotlin.Any? declared in <root>.C.<init>' type=kotlin.Any? origin=null
+    FUN FAKE_OVERRIDE name:equals visibility:public modality:OPEN <> ($this:kotlin.Any, other:kotlin.Any?) returnType:kotlin.Boolean
+      overridden:
+        public open fun equals (other: kotlin.Any?): kotlin.Boolean declared in kotlin.Any
+      $this: VALUE_PARAMETER name:<this> type:kotlin.Any
+      VALUE_PARAMETER name:other index:0 type:kotlin.Any?
+    FUN FAKE_OVERRIDE name:hashCode visibility:public modality:OPEN <> ($this:kotlin.Any) returnType:kotlin.Int
+      overridden:
+        public open fun hashCode (): kotlin.Int declared in kotlin.Any
+      $this: VALUE_PARAMETER name:<this> type:kotlin.Any
+    FUN FAKE_OVERRIDE name:toString visibility:public modality:OPEN <> ($this:kotlin.Any) returnType:kotlin.String
+      overridden:
+        public open fun toString (): kotlin.String declared in kotlin.Any
+      $this: VALUE_PARAMETER name:<this> type:kotlin.Any