Do not ignore copy operations while finding source instructions

in redundant locals elimination.

Since IgnoringCopySourceInterpreter ignores ALOADs and ASTOREs,
the source instruction of ALOAD K in {ALOAD N, ASTORE K, ALOAD K}
sequence is not ASTORE K. In this case we cannot simply replace K
with N, since there can be multiple {ALOAD N, ASTORE K} sequences
in separate branches. After replacement we get different stack
frames.
This change resolves this.

However, in ReturnUnitMethodTransformer we want to ignore copies
of the same GETSTATIC kotlin/Unit.INSTANCE, since we do not mess
with local variables and just replace ASTORE with ARETURN to help
tail-call optimization.
 #KT-23373: Fixed

compiler/testData/codegen/box/coroutines/redundantLocalsElimination/ktor_receivedMessage.kt

@@ -0,0 +1,43 @@
+// WITH_RUNTIME
+// WITH_COROUTINES
+
+import helpers.*
+import kotlin.coroutines.experimental.*
+
+private var prevSender: String = "FAIL"
+
+class ChatServer {
+    suspend fun who(sender: String) {
+        prevSender = sender
+    }
+    suspend fun sendTo(recipient: String, sender: String, message: String) { }
+    suspend fun message(sender: String, message: String) { }
+}
+
+private val server = ChatServer()
+
+private suspend fun receivedMessage(id: String, command: String) {
+    when {
+        command.startsWith("/who") -> server.who(id)
+        command.startsWith("/user") -> {
+            val newName = command.removePrefix("/user").trim()
+            when {
+                newName.isEmpty() -> server.sendTo(id, "server::help", "/user [newName]")
+                else -> server.message(id, newName)
+            }
+        }
+        command.startsWith("/") -> server.sendTo(id, "server::help", "Unknown command ${command.takeWhile { !it.isWhitespace() }}")
+        else -> server.message(id, command)
+    }
+}
+
+fun builder(c: suspend () -> Unit) {
+    c.startCoroutine(EmptyContinuation)
+}
+
+fun box(): String {
+    builder {
+        receivedMessage("OK", "/who")
+    }
+    return prevSender
+}