[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()