Fir2Ir: generate coercion to Unit for when branches if needed

Before this change, it could happen that `when` of type Unit has a
branch whose type is not Unit. This can lead to problems in IR
lowerings, for example PolymorphicSignatureLowering which is very
reliant on the correct types of expressions and placement of coercions
to Unit (KT-59218).

compiler/testData/ir/irText/expressions/ifElseIf.fir.kt.txt

@@ -24,7 +24,7 @@ fun testEmptyBranches2(flag: Boolean) {
     else -> true
   } /*~> Unit */
   when {
-    flag -> true
+    flag -> true /*~> Unit */
     else -> { // BLOCK
     }
   }
@@ -37,3 +37,4 @@ fun testEmptyBranches3(flag: Boolean) {
     else -> true
   } /*~> Unit */
 }
+