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/expressions/coercionToUnit.fir.ir.txt

@@ -2,8 +2,8 @@ FILE fqName:<root> fileName:/coercionToUnit.kt
   PROPERTY name:test1 visibility:public modality:FINAL [val]
     FIELD PROPERTY_BACKING_FIELD name:test1 type:kotlin.Function0<kotlin.Unit> visibility:private [final,static]
       EXPRESSION_BODY
-        FUN_EXPR type=kotlin.Function0<kotlin.Int> origin=LAMBDA
-          FUN LOCAL_FUNCTION_FOR_LAMBDA name:<anonymous> visibility:local modality:FINAL <> () returnType:kotlin.Int
+        FUN_EXPR type=kotlin.Function0<kotlin.Unit> origin=LAMBDA
+          FUN LOCAL_FUNCTION_FOR_LAMBDA name:<anonymous> visibility:local modality:FINAL <> () returnType:kotlin.Unit
             BLOCK_BODY
               TYPE_OP type=kotlin.Unit origin=IMPLICIT_COERCION_TO_UNIT typeOperand=kotlin.Unit
                 CONST Int type=kotlin.Int value=42

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

@@ -1,5 +1,5 @@
 val test1: Function0<Unit>
-  field = local fun <anonymous>(): Int {
+  field = local fun <anonymous>() {
     42 /*~> Unit */
   }
 

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
   }
 )
 }