Remove Concat and StringPlus intrinsics in favor of IrStringConcatenation.

compiler/testData/codegen/bytecodeText/stringOperations/stringPlus.kt

@@ -0,0 +1,17 @@
+// IGNORE_BACKEND: JVM
+
+fun f(s: String?, t: String): String {
+    return s.plus(t)
+}
+
+fun g(s: String, t: Any?): String {
+    return "$s$t"
+}
+
+fun h(s: String, t: Any?): String {
+    return s + t
+}
+
+// 0 valueOf
+// 0 NEW java/lang/StringBuilder
+// 3 INVOKESTATIC kotlin/jvm/internal/Intrinsics.stringPlus