[FIR2IR] Don't replace while loop and when branch body blocks with single expression. Skip some empty blocks.

https://youtrack.jetbrains.com/issue/KT-60264/K2-while-loop-body-block-sometimes-replaced-with-single-expression

Merge-request: KT-MR-12035
Merged-by: Vladimir Sukharev <Vladimir.Sukharev@jetbrains.com>

compiler/testData/ir/irText/firProblems/OutBox.fir.kt.txt

@@ -13,9 +13,12 @@ class Box<out T : Foo> {
     get(): String {
       var foo: Foo = <this>.<get-foo>()
       when {
-        foo is Buz -> return foo /*as Buz */.#str /*!! String */
+        foo is Buz -> { // BLOCK
+          return foo /*as Buz */.#str /*!! String */
+        }
       }
       return ""
     }
 
 }
+