SI-8601 Don't treat newarray as dead code

Otherwise we lose the side effect of a `NegativeArraySizeException`.

A test for this case already exists (run/t8601b.scala), but it currently
enforces `-optimize -Ybackend:GenASM`, so it didn't trigger on the new
backend. However, PR scala#4814 was merged into 2.12.x and moved that test
over to the new backend and optimizer.  After merging the 2.12.x into
the current optimizer branch (push-pop elimination), the test started
failing.

Also disable the optimizer for `jvm/bytecode-test-example`: it counts
the number of null checks in a method, the optimizer (rightly) eliminates
one of the two.

Author: lrytz

src/compiler/scala/tools/nsc/backend/jvm/opt/CopyProp.scala

@@ -382,10 +382,6 @@ class CopyProp[BT <: BTypes](val btypes: BT) {
               popAfterProd()
           }
 
-          case NEWARRAY | ANEWARRAY =>
-            toRemove += prod
-            handleInputs(prod, 1)
-
           case MULTIANEWARRAY =>
             toRemove += prod
             handleInputs(prod, prod.asInstanceOf[MultiANewArrayInsnNode].dims)

test/files/jvm/bytecode-test-example.flags

@@ -0,0 +1 @@
+-Yopt:l:none

test/junit/scala/tools/nsc/backend/jvm/opt/MethodLevelOptsTest.scala

@@ -161,15 +161,15 @@ class MethodLevelOptsTest extends ClearAfterClass {
         |    val b = (a, y)                            // Tuple2
         |    val c = (new Object, "krik", new String)  // unused java/lang/Object, java/lang/String allocation and string constant is also eliminated
         |    val d = new java.lang.Integer(x)
-        |    val e = new String(new Array[Char](23))
+        |    val e = new String(new Array[Char](23))   // array allocation not eliminated, as it may throw (negative size, SI-8601)
         |    val f = new scala.runtime.IntRef(11)
         |    x + y
         |  }
         |}
       """.stripMargin
     val List(c) = compileClasses(methodOptCompiler)(code)
     assertEquals(getSingleMethod(c, "t").instructions.dropNonOp,
-      List(VarOp(ILOAD, 1), VarOp(ILOAD, 2), Op(IADD), Op(IRETURN)))
+      List(IntOp(BIPUSH, 23), IntOp(NEWARRAY, 5), Op(POP), VarOp(ILOAD, 1), VarOp(ILOAD, 2), Op(IADD), Op(IRETURN)))
   }
 
   @Test