FIR: pass the explicit expected type to block type

This helps avoid adding redundant return Unit into block.

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

@@ -1,6 +1,6 @@
 val test1: Function0<Unit>
   field = local fun <anonymous>(): Int {
-    return 42
+    42 /*~> Unit */
   }
 
   get

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

@@ -5,7 +5,7 @@ FILE fqName:<root> fileName:/coercionToUnit.kt
         FUN_EXPR type=kotlin.Function0<kotlin.Int> origin=LAMBDA
           FUN LOCAL_FUNCTION_FOR_LAMBDA name:<anonymous> visibility:local modality:FINAL <> () returnType:kotlin.Int
             BLOCK_BODY
-              RETURN type=kotlin.Nothing from='local final fun <anonymous> (): kotlin.Int declared in <root>.test1'
+              TYPE_OP type=kotlin.Unit origin=IMPLICIT_COERCION_TO_UNIT typeOperand=kotlin.Unit
                 CONST Int type=kotlin.Int value=42
     FUN DEFAULT_PROPERTY_ACCESSOR name:<get-test1> visibility:public modality:FINAL <> () returnType:kotlin.Function0<kotlin.Unit>
       correspondingProperty: PROPERTY name:test1 visibility:public modality:FINAL [val]