[FIR2IR] Coerce last operator in a loop to Unit

One of many fixes for https://youtrack.jetbrains.com/issue/KT-59781/K2-investigate-implicit-cast-generation-in-fir2ir-vs-psi2ir

Merge-request: KT-MR-12629
Merged-by: Vladimir Sukharev <Vladimir.Sukharev@jetbrains.com>

compiler/testData/ir/irText/firProblems/DeepCopyIrTree.fir.kt.txt

@@ -64,7 +64,7 @@ class DeepCopyIrTreeWithSymbols {
               return <this>.<get-typeRemapper>().remapType(type = it)
             }
 )
-          }
+          } /*~> Unit */
         }
       }
     }