[FIR2IR] Fix unwrapping of single-statement blocks

Unwrapping should be done with respect to `forceUnitType` parameter.

^KT-65019: Fixed

compiler/testData/codegen/box/finally/decrementInFinally.kt

@@ -0,0 +1,12 @@
+fun box(): String {
+    var variable = 0
+    try {
+        try {
+            null!!
+        } finally {
+            variable--
+        }
+    } catch (e: NullPointerException) {
+        return if (variable == -1) "OK" else "Fail"
+    }
+}