[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/fir/raw-fir/light-tree2fir/src/org/jetbrains/kotlin/fir/lightTree/converter/AbstractLightTreeRawFirBuilder.kt

@@ -90,7 +90,7 @@ abstract class AbstractLightTreeRawFirBuilder(
         }
     }
 
-    private fun LighterASTNode.getLastChildExpression(): LighterASTNode? {
+    fun LighterASTNode.getLastChildExpression(): LighterASTNode? {
         var result: LighterASTNode? = null
         forEachChildren {
             if (it.isExpression()) {

compiler/fir/raw-fir/light-tree2fir/src/org/jetbrains/kotlin/fir/lightTree/converter/LightTreeRawFirExpressionBuilder.kt

@@ -1402,13 +1402,19 @@ class LightTreeRawFirExpressionBuilder(
                 parent = parent.getParent() ?: return true
             }
             val parentTokenType = parent.tokenType
-            if (parentTokenType == BLOCK) return false
-            if (parentTokenType == THEN || parentTokenType == ELSE || parentTokenType == WHEN_ENTRY) {
-                return parent.getParent()?.usedAsExpression ?: true
+            return when (parentTokenType) {
+                BLOCK -> parent.getLastChildExpression() == this && parent.usedAsExpression
+                TRY, CATCH -> parent.usedAsExpression
+                THEN, ELSE, WHEN_ENTRY -> parent.getParent()?.usedAsExpression ?: true
+                CLASS_INITIALIZER, SCRIPT_INITIALIZER, SECONDARY_CONSTRUCTOR, FUNCTION_LITERAL, FINALLY -> false
+                FUN, PROPERTY_ACCESSOR -> parent.getChildrenAsArray().any { it?.tokenType == EQ }
+                DOT_QUALIFIED_EXPRESSION -> parent.getFirstChild() == this
+                BODY -> when (parent.getParent()?.tokenType) {
+                    FOR, WHILE, DO_WHILE -> false
+                    else -> true
+                }
+                else -> true
             }
-            if (parentTokenType != BODY) return true
-            val type = parent.getParent()?.tokenType ?: return true
-            return !(type == FOR || type == WHILE || type == DO_WHILE)
         }
 
     /**

compiler/fir/raw-fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/PsiRawFirBuilder.kt

@@ -2690,17 +2690,33 @@ open class PsiRawFirBuilder(
                 ) {
                     parent = parent.parent
                 }
-                if (parent is KtBlockExpression) return false
+
                 when (parent.elementType) {
                     KtNodeTypes.THEN, KtNodeTypes.ELSE, KtNodeTypes.WHEN_ENTRY -> {
                         return (parent.parent as? KtExpression)?.usedAsExpression != false
                     }
                 }
-                if (parent is KtScriptInitializer) return false
-                // Here we check that when used is a single statement of a loop
-                if (parent !is KtContainerNodeForControlStructureBody) return true
-                val type = parent.parent.elementType
-                return !(type == KtNodeTypes.FOR || type == KtNodeTypes.WHILE || type == KtNodeTypes.DO_WHILE)
+
+                fun PsiElement.getLastChildExpression() = children.asList().asReversed().firstIsInstanceOrNull<KtExpression>()
+
+                return when (parent) {
+//                  todo KT-62472 Replace with the following when K2 is used in TeamCity
+//                    is KtBlockExpression, is KtTryExpression -> parent.getLastChildExpression() == this && parent.usedAsExpression
+                    is KtBlockExpression -> parent.getLastChildExpression() == this && parent.usedAsExpression
+                    is KtTryExpression -> parent.getLastChildExpression() == this && parent.usedAsExpression
+                    is KtCatchClause -> (parent.parent as? KtTryExpression)?.usedAsExpression == true
+                    is KtClassInitializer, is KtScriptInitializer, is KtSecondaryConstructor, is KtFunctionLiteral, is KtFinallySection -> false
+                    is KtDotQualifiedExpression -> parent.firstChild == this
+//                  todo KT-62472 Replace with the following when K2 is used in TeamCity
+//                    is KtFunction, is KtPropertyAccessor -> parent.hasBody() && !parent.hasBlockBody()
+                    is KtFunction -> parent.hasBody() && !parent.hasBlockBody()
+                    is KtPropertyAccessor -> parent.hasBody() && !parent.hasBlockBody()
+                    is KtContainerNodeForControlStructureBody -> when (parent.parent.elementType) {
+                        KtNodeTypes.FOR, KtNodeTypes.WHILE, KtNodeTypes.DO_WHILE -> false
+                        else -> true
+                    }
+                    else -> true
+                }
             }
 
         override fun visitDoWhileExpression(expression: KtDoWhileExpression, data: FirElement?): FirElement {