[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/when/nonExhaustiveDependentContext.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // SKIP_TXT
 fun bar(a: String): String {
     return when {