Add benchmark for plusAssign desugaring

#KT-38101

benchmarks/src/org/jetbrains/kotlin/benchmarks/PlusAssignOperatorDesugaringBenchmark.kt

@@ -0,0 +1,48 @@
+/*
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.benchmarks
+
+import org.openjdk.jmh.annotations.*
+import org.openjdk.jmh.infra.Blackhole
+import java.util.concurrent.TimeUnit
+
+@BenchmarkMode(Mode.AverageTime)
+@OutputTimeUnit(TimeUnit.MILLISECONDS)
+@State(Scope.Benchmark)
+open class PlusAssignOperatorDesugaringBenchmark : AbstractInferenceBenchmark() {
+    @Param("9", "10", "11", "12", "13", "14")
+    private var size: Int = 0
+
+    @Benchmark
+    fun benchmark(bh: Blackhole) {
+        analyzeGreenFile(bh)
+    }
+
+    override fun buildText(): String = buildString {
+        appendln(
+            """
+            class A {
+                operator fun <T : Number> plus(other: (Int) -> T): A = this
+                operator fun <T : CharSequence> plusAssign(other: (String) -> T) {}
+            }
+        """.trimIndent()
+        )
+        appendln("fun test() {")
+        appendln("var a = A()")
+        for (i in 1..size) {
+            appendln("a += {")
+        }
+        for (i in 1..size) {
+            appendln(
+                """
+                it.inc()
+                1
+                }
+            """.trimIndent())
+        }
+        appendln()
+    }
+}