[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/expressions/breakContinueInLoopHeader.fir.kt.txt

@@ -69,7 +69,7 @@ fun test5() {
       { // BLOCK
         j = j.inc()
         j
-      }
+      } /*~> Unit */
       // }      while (when {
         greaterOrEqual(arg0 = j, arg1 = 3) -> false
         else -> break@Inner