SI-6288 Fix positioning of label jumps

ICode generation was assigning the position of
the last label jump to all jumps to that particular
label def.

This problem is particularly annoying under the new
pattern matcher: a breakpoint in the body of the final
case will be triggered on the way out of the body of
any other case.

Thanks to @dragos for the expert guidance as we
wended our way through GenICode to the troublesome
code. Chalk up another bug for mutability.

I believe that the ICode output should be stable
enough to use a a .check file, if it proves otherwise
we should make it so.

src/compiler/scala/tools/nsc/backend/icode/BasicBlocks.scala

@@ -320,7 +320,12 @@ trait BasicBlocks {
       else
         instrs.zipWithIndex collect {
           case (oldInstr, i) if map contains oldInstr =>
-            code.touched |= replaceInstruction(i, map(oldInstr))
+            // SI-6288 clone important here because `replaceInstruction` assigns
+            // a position to `newInstr`. Without this, a single instruction can
+            // be added twice, and the position last position assigned clobbers
+            // all previous positions in other usages.
+            val newInstr = map(oldInstr).clone()
+            code.touched |= replaceInstruction(i, newInstr)
         }
 
     ////////////////////// Emit //////////////////////

src/partest/scala/tools/partest/DirectTest.scala

@@ -39,6 +39,10 @@ abstract class DirectTest extends App {
   // new compiler
   def newCompiler(args: String*): Global = {
     val settings = newSettings((CommandLineParser tokenize ("-d \"" + testOutput.path + "\" " + extraSettings)) ++ args.toList)
+    newCompiler(settings)
+  }
+
+  def newCompiler(settings: Settings): Global = {
     if (settings.Yrangepos.value) new Global(settings, reporter(settings)) with interactive.RangePositions
     else new Global(settings, reporter(settings))
   }

test/files/run/t6288b-jump-position.check

@@ -0,0 +1,80 @@
+object Case3 extends Object {
+  // fields:
+
+  // methods
+  def unapply(z: Object (REF(class Object))): Option {
+  locals: value z
+  startBlock: 1
+  blocks: [1]
+
+  1: 
+    2	NEW REF(class Some)
+    2	DUP(REF(class Some))
+    2	CONSTANT(-1)
+    2	BOX INT
+    2	CALL_METHOD scala.Some.<init> (static-instance)
+    2	RETURN(REF(class Option))
+
+  }
+  Exception handlers: 
+
+  def main(args: Array[String] (ARRAY[REF(class String)])): Unit {
+  locals: value args, value x1, value x2, value x
+  startBlock: 1
+  blocks: [1,2,3,6,7]
+
+  1: 
+    4	CONSTANT("")
+    4	STORE_LOCAL(value x1)
+    4	SCOPE_ENTER value x1
+    4	JUMP 2
+
+  2: 
+    5	LOAD_LOCAL(value x1)
+    5	IS_INSTANCE REF(class String)
+    5	CZJUMP (BOOL)NE ? 3 : 6
+
+  3: 
+    5	LOAD_LOCAL(value x1)
+    5	CHECK_CAST REF(class String)
+    5	STORE_LOCAL(value x2)
+    5	SCOPE_ENTER value x2
+    6	LOAD_MODULE object Predef
+    6	CONSTANT("case 0")
+    6	CALL_METHOD scala.Predef.println (dynamic)
+    6	LOAD_FIELD scala.runtime.BoxedUnit.UNIT
+    6	STORE_LOCAL(value x)
+    6	JUMP 7
+
+  6: 
+    8	LOAD_MODULE object Predef
+    8	CONSTANT("default")
+    8	CALL_METHOD scala.Predef.println (dynamic)
+    8	LOAD_FIELD scala.runtime.BoxedUnit.UNIT
+    8	STORE_LOCAL(value x)
+    8	JUMP 7
+
+  7: 
+    10	LOAD_MODULE object Predef
+    10	CONSTANT("done")
+    10	CALL_METHOD scala.Predef.println (dynamic)
+    10	RETURN(UNIT)
+
+  }
+  Exception handlers: 
+
+  def <init>(): Case3.type {
+  locals: 
+  startBlock: 1
+  blocks: [1]
+
+  1: 
+    12	THIS(Case3)
+    12	CALL_METHOD java.lang.Object.<init> (super())
+    12	RETURN(UNIT)
+
+  }
+  Exception handlers: 
+
+
+}

test/files/run/t6288b-jump-position.scala

@@ -0,0 +1,22 @@
+import scala.tools.partest.IcodeTest
+
+object Test extends IcodeTest {
+  override def code =
+    """object Case3 {                                // 01
+     |  def unapply(z: Any): Option[Int] = Some(-1)  // 02
+     |  def main(args: Array[String]) {              // 03
+     |    ("": Any) match {                          // 04
+     |      case x : String =>                       // 05 Read: <linenumber> JUMP <target basic block id>
+     |        println("case 0")                      // 06 expecting "6 JUMP 7", was "8 JUMP 7"
+     |      case _ =>                                // 07
+     |        println("default")                     // 08 expecting "8 JUMP 7"
+     |    }                                          // 09
+     |    println("done")                            // 10
+     |  }
+     |}""".stripMargin
+
+  override def show() {
+    val lines1 = collectIcode("")
+    println(lines1 mkString "\n")
+  }
+}