[IR] Avoid unnecessary getting of returnType, when not needed for mangling

^KT-73110
diff --git a/compiler/ir/serialization.common/src/org/jetbrains/kotlin/backend/common/serialization/mangle/BaseKotlinMangleComputer.kt b/compiler/ir/serialization.common/src/org/jetbrains/kotlin/backend/common/serialization/mangle/BaseKotlinMangleComputer.kt
index ef198ef..336cc68 100644
--- a/compiler/ir/serialization.common/src/org/jetbrains/kotlin/backend/common/serialization/mangle/BaseKotlinMangleComputer.kt
+++ b/compiler/ir/serialization.common/src/org/jetbrains/kotlin/backend/common/serialization/mangle/BaseKotlinMangleComputer.kt
@@ -191,9 +191,11 @@
             mangleTypeParameter(this, it.value, it.index, session)
         }
 
-        getReturnType(this)?.let {
-            if (!isConstructor && !isUnit(it) && (addReturnType() || addReturnTypeSpecialCase(this@mangleSignature))) {
-                mangleType(builder, it, session)
+        if (!isConstructor && (addReturnType() || addReturnTypeSpecialCase(this@mangleSignature))) {
+            getReturnType(this)?.let {
+                if (!isUnit(it)) {
+                    mangleType(builder, it, session)
+                }
             }
         }