Make coercion to Unit explicit in IR.

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

@@ -0,0 +1,34 @@
+FILE /safeAssignment.kt
+  CLASS CLASS C
+    CONSTRUCTOR public constructor C(x: kotlin.Int)
+      BLOCK_BODY
+        DELEGATING_CONSTRUCTOR_CALL 'constructor Any()'
+        INSTANCE_INITIALIZER_CALL classDescriptor='C'
+    PROPERTY public final var x: kotlin.Int
+      FIELD PROPERTY_BACKING_FIELD public final var x: kotlin.Int
+        EXPRESSION_BODY
+          GET_VAR 'value-parameter x: Int' type=kotlin.Int origin=INITIALIZE_PROPERTY_FROM_PARAMETER
+      FUN DEFAULT_PROPERTY_ACCESSOR public final fun <get-x>(): kotlin.Int
+        BLOCK_BODY
+          RETURN type=kotlin.Nothing from='<get-x>(): Int'
+            GET_FIELD 'x: Int' type=kotlin.Int origin=null
+              receiver: THIS of 'C' type=C
+      FUN DEFAULT_PROPERTY_ACCESSOR public final fun <set-x>(<set-?>: kotlin.Int): kotlin.Unit
+        BLOCK_BODY
+          SET_FIELD 'x: Int' type=kotlin.Unit origin=null
+            receiver: THIS of 'C' type=C
+            value: GET_VAR 'value-parameter <set-?>: Int' type=kotlin.Int origin=null
+  FUN public fun test(nc: C?): kotlin.Unit
+    BLOCK_BODY
+      BLOCK type=kotlin.Unit origin=SAFE_CALL
+        VAR IR_TEMPORARY_VARIABLE val tmp0_safe_receiver: C?
+          GET_VAR 'value-parameter nc: C?' type=C? origin=null
+        WHEN type=kotlin.Unit origin=SAFE_CALL
+          if: CALL 'EQEQ(Any?, Any?): Boolean' type=kotlin.Boolean origin=EQEQ
+            arg0: GET_VAR 'tmp0_safe_receiver: C?' type=C? origin=null
+            arg1: CONST Null type=kotlin.Nothing? value='null'
+          then: TYPE_OP origin=IMPLICIT_COERCION_TO_UNIT typeOperand=kotlin.Unit
+            CONST Null type=kotlin.Nothing? value='null'
+          else: CALL '<set-x>(Int): Unit' type=kotlin.Unit origin=EQ
+            $this: GET_VAR 'tmp0_safe_receiver: C?' type=C? origin=null
+            <set-?>: CONST Int type=kotlin.Int value='42'