[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/FunctionCalleeExpressions.fir.kt

@@ -71,7 +71,7 @@ fun main1() {
 
     1.<!ILLEGAL_SELECTOR!>"sdf"<!>
     1.<!ILLEGAL_SELECTOR!>{}<!>
-    1.<!ILLEGAL_SELECTOR!><!INVALID_IF_AS_EXPRESSION!>if<!> (true) {}<!>
+    1.<!ILLEGAL_SELECTOR!>if (true) {}<!>
 }
 
 fun test() {