FIR: Fix incorrect type of block generated for inc operator

Previously, it was obtained from expected type of a variable being assigned,
but it's better to use the type of resulting expression

Initially this part was brought in 4ab0897d7d,
but as we see in commit message and tests it was all about unit-coercion

compiler/testData/ir/irText/lambdas/lambdaReturningUnit.fir.ir.txt

@@ -11,5 +11,4 @@ FILE fqName:<root> fileName:/lambdaReturningUnit.kt
         block: FUN_EXPR type=kotlin.Function0<kotlin.Any?> origin=LAMBDA
           FUN LOCAL_FUNCTION_FOR_LAMBDA name:<anonymous> visibility:local modality:FINAL <> () returnType:kotlin.Any?
             BLOCK_BODY
-              RETURN type=kotlin.Nothing from='local final fun <anonymous> (): kotlin.Any? declared in <root>.box'
-                GET_OBJECT 'CLASS IR_EXTERNAL_DECLARATION_STUB OBJECT name:Unit modality:FINAL visibility:public superTypes:[kotlin.Any]' type=kotlin.Unit
+              GET_OBJECT 'CLASS IR_EXTERNAL_DECLARATION_STUB OBJECT name:Unit modality:FINAL visibility:public superTypes:[kotlin.Any]' type=kotlin.Unit

compiler/testData/ir/irText/lambdas/lambdaReturningUnit.fir.kt.txt

@@ -4,7 +4,7 @@ inline fun flaf(block: Function0<Any?>) {
 
 suspend fun box() {
   flaf(block = local fun <anonymous>(): Any? {
-    return Unit
+    Unit
   }
 )
 }