[FIR] Remove implicit coercion of ifs and whens to Unit

The rule of thumb is the following:
If the `if` and `when` can be successfully replaced with `while`,
then it is used as a statement, otherwise, it is used as an expression.

#KT-59883

compiler/testData/diagnostics/tests/controlStructures/ifWhenToAnyComplexExpressions.fir.kt

@@ -18,7 +18,7 @@ fun testMixedIfAndWhen() =
 fun testWrappedExpressions() =
         if (true) {
             println()
-            if (true) {
+            <!INVALID_IF_AS_EXPRESSION!>if<!> (true) {
                 println()
                 if (true) {
                     println()

compiler/testData/diagnostics/tests/controlStructures/kt4310.fir.kt

@@ -1,11 +1,11 @@
 package f
 
 fun test(a: Boolean, b: Boolean): Int {
-    return <!RETURN_TYPE_MISMATCH!>if(a) {
+    return if(a) {
         1
     } else {
-        if (b) {
+        <!INVALID_IF_AS_EXPRESSION!>if<!> (b) {
             3
         }
-    }<!>
+    }
 }