[IR] Handled degenerate cases of IrStringConcatenation

It is not needed to create StringBuilder if the number of values
being concatenated is 0 or 1

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/lower/StringConcatenationLowering.kt

@@ -20,14 +20,12 @@ import org.jetbrains.kotlin.backend.common.CommonBackendContext
 import org.jetbrains.kotlin.backend.common.FileLoweringPass
 import org.jetbrains.kotlin.backend.common.atMostOne
 import org.jetbrains.kotlin.ir.IrStatement
-import org.jetbrains.kotlin.ir.builders.IrBuilderWithScope
-import org.jetbrains.kotlin.ir.builders.createTmpVariable
-import org.jetbrains.kotlin.ir.builders.irCall
-import org.jetbrains.kotlin.ir.builders.irGet
+import org.jetbrains.kotlin.ir.builders.*
 import org.jetbrains.kotlin.ir.declarations.*
 import org.jetbrains.kotlin.ir.expressions.IrExpression
 import org.jetbrains.kotlin.ir.expressions.IrStringConcatenation
 import org.jetbrains.kotlin.ir.types.IrType
+import org.jetbrains.kotlin.ir.types.isNullable
 import org.jetbrains.kotlin.ir.types.isNullableAny
 import org.jetbrains.kotlin.ir.util.constructors
 import org.jetbrains.kotlin.ir.util.functions
@@ -88,17 +86,36 @@ private class StringConcatenationTransformer(val lower: StringConcatenationLower
 
         expression.transformChildrenVoid(this)
         val blockBuilder = buildersStack.last()
-        return blockBuilder.irBlock(expression) {
-            val stringBuilderImpl = createTmpVariable(irCall(constructor))
-            expression.arguments.forEach { arg ->
-                val appendFunction = typeToAppendFunction(arg.type)
-                +irCall(appendFunction).apply {
-                    dispatchReceiver = irGet(stringBuilderImpl)
-                    putValueArgument(0, arg)
+
+        val arguments = expression.arguments
+        return when (arguments.size) {
+            0 -> blockBuilder.irString("")
+            1 -> {
+                val argument = arguments[0]
+                if (argument.type.isNullable())
+                    blockBuilder.irCall(context.ir.symbols.extensionToString).apply {
+                        extensionReceiver = argument
+                    }
+                else blockBuilder.irCall(
+                    context.irBuiltIns.anyClass.functions
+                        .single { it.owner.name.asString() == "toString" }).apply {
+                    dispatchReceiver = argument
                 }
             }
-            +irCall(toStringFunction).apply {
-                dispatchReceiver = irGet(stringBuilderImpl)
+            else -> {
+                blockBuilder.irBlock(expression) {
+                    val stringBuilderImpl = createTmpVariable(irCall(constructor))
+                    expression.arguments.forEach { arg ->
+                        val appendFunction = typeToAppendFunction(arg.type)
+                        +irCall(appendFunction).apply {
+                            dispatchReceiver = irGet(stringBuilderImpl)
+                            putValueArgument(0, arg)
+                        }
+                    }
+                    +irCall(toStringFunction).apply {
+                        dispatchReceiver = irGet(stringBuilderImpl)
+                    }
+                }
             }
         }
     }