FIR: Do not use return statement for type of a block expression

Type of a block is a kind of irrelevant for lambdas: their type is much
more complicated and defined via FirDataFlowAnalyzer#returnExpressionsOfAnonymousFunction
at at FirCallCompleter.LambdaAnalyzerImpl#analyzeAndGetLambdaReturnArguments

compiler/fir/analysis-tests/testData/resolve/inference/lambdasReturns.fir.txt

@@ -7,7 +7,7 @@ FILE: lambdasReturns.kt
     }
     public final fun foo(x: R|kotlin/String?|): R|kotlin/Unit| {
         lval r: R|kotlin/Int| = R|/myRun|<R|kotlin/Int|>(<L> = myRun@fun <anonymous>(): R|kotlin/Int| {
-            lval y: R|kotlin/String| = R|<local>/x| ?: ^@myRun R?C|/materialize|()
+            lval y: R|kotlin/String| = R|<local>/x| ?: ^@myRun R|/materialize|<R|kotlin/Int|>()
             ^ R|<local>/y|.R|kotlin/String.length|
         }
         )