[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/kt4434.fir.kt

@@ -1,37 +0,0 @@
-/*
- * KOTLIN DIAGNOSTICS SPEC TEST (POSITIVE)
- *
- * SPEC VERSION: 0.1-152
- * PRIMARY LINKS: expressions, when-expression -> paragraph 5 -> sentence 1
- * expressions, when-expression, exhaustive-when-expressions -> paragraph 1 -> sentence 1
- * expressions, when-expression, exhaustive-when-expressions -> paragraph 2 -> sentence 1
- * expressions, when-expression -> paragraph 6 -> sentence 5
- */
-
-// KT-4434 Missed diagnostic about else branch in when
-
-package test
-
-fun foo(): Int {
-    val a = "a"
-    return <!RETURN_TYPE_MISMATCH!>if (a.length > 0) {
-        when (a) {
-            "a" -> 1
-        }
-    }
-    else {
-        3
-    }<!>
-}
-
-fun bar(): Int {
-    val a = "a"
-    if (a.length > 0) {
-        return <!NO_ELSE_IN_WHEN!>when<!> (a) {
-            "a" -> 1
-        }
-    }
-    else {
-        return 3
-    }
-}

compiler/testData/diagnostics/tests/when/kt4434.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 /*
  * KOTLIN DIAGNOSTICS SPEC TEST (POSITIVE)
  *

compiler/testData/diagnostics/tests/when/nonExhaustiveDependentContext.fir.kt

@@ -1,13 +0,0 @@
-// SKIP_TXT
-fun bar(a: String): String {
-    return <!RETURN_TYPE_MISMATCH!>when {
-        a.length == 1 -> {
-            when { // Error in K1, no error in K2
-                a == "a" -> ""
-                a == "b" -> ""
-            }
-        }
-
-        else -> ""
-    }<!>
-}

compiler/testData/diagnostics/tests/when/nonExhaustiveDependentContext.kt

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