SI-8601 Don't treat int/long division, or arraylength, as dead-code

`{i, l}div` and `{i, l}rem` throw an `ArithmeticException` if the
divisor is 0.

`arraylength` throws a `NullPointerException` on a null reference.

JVM Spec:
> The only integer operations that can throw an exception are the
> integer divide instructions (idiv and ldiv) and the integer
> remainder instructions (irem and lrem), which throw an
> ArithmeticException if the divisor is zero.

> The Java virtual machine's floating-point operators do not throw
> runtime exceptions

> If the arrayref is null, the arraylength instruction throws a
> NullPointerException.

I checked the other primitives in `ICode` to see if anything else
should be considered as live code.

Pure:

	  // jvm  : {i, l, f, d}neg
	  case class Negation(kind: TypeKind) extends Primitive
	  // jvm  : if{eq, ne, lt, ge, le, gt}, if{null, nonnull}
	  //        if_icmp{eq, ne, lt, ge, le, gt}, if_acmp{eq,ne}
	  case class Test(op: TestOp, kind: TypeKind,  zero: Boolean)  extends Primitive
	  // jvm  : lcmp, {f, d}cmp{l, g}
	  case class Comparison(op: ComparisonOp, kind: TypeKind) extends Primitive

Impure: {i, l}{div, rem}, otherwise pure

	  // jvm  : {i, l, f, d}{add, sub, mul, div, rem}
	  case class Arithmetic(op: ArithmeticOp, kind: TypeKind) extends Primitive

Pure (overflow is silent, NaN.toInt is defined):
	  // jvm  : {i, l}{and, or, xor}
	  case class Logical(op: LogicalOp, kind: TypeKind) extends Primitive

	  // jvm  : {i, l}{shl, ushl, shr}
	  case class Shift(op: ShiftOp, kind: TypeKind) extends Primitive

	  // jvm  : i2{l, f, d}, l2{i, f, d}, f2{i, l, d}, d2{i, l, f}, i2{b, c, s}
	  case class Conversion(src: TypeKind, dst: TypeKind) extends Primitive

Impure! May NPE!

	  // jvm  : arraylength
	  case class ArrayLength(kind: TypeKind) extends Primitive

Pure (we know that StringBuilder.{<init>, append, toString} are pure
and `append` is null safe.)

	  // jvm  : It should call the appropiate 'append' method on StringBuffer
	  case class StringConcat(el: TypeKind) extends Primitive

	  // jvm:  it should create a new StringBuffer
	  case object StartConcat extends Primitive

      // jvm:  convert StringBuffer to a String
	  case object EndConcat extends Primitive

Author: retronym

src/compiler/scala/tools/nsc/backend/opt/DeadCodeElimination.scala

@@ -193,6 +193,8 @@ abstract class DeadCodeElimination extends SubComponent {
               moveToWorkListIf(necessary)
             case LOAD_MODULE(sym) if isLoadNeeded(sym) =>
               moveToWorkList() // SI-4859 Module initialization might side-effect.
+            case CALL_PRIMITIVE(Arithmetic(DIV | REM, INT | LONG) | ArrayLength(_)) =>
+              moveToWorkList() // SI-8601 Might divide by zero
             case _ => ()
               moveToWorkListIf(cond = false)
           }

test/files/run/t8601.flags

@@ -0,0 +1 @@
+-optimize

test/files/run/t8601.scala

@@ -0,0 +1,15 @@
+object Test {
+  def idiv(x: Int): Unit = x / 0
+  def ldiv(x: Long): Unit = x / 0
+  def irem(x: Int): Unit = x % 0
+  def lrem(x: Long): Unit = x % 0
+
+  def check(x: => Any) = try { x; sys.error("failed to throw divide by zero!") } catch { case _: ArithmeticException => }
+
+  def main(args: Array[String]) {
+    check(idiv(1))
+    check(ldiv(1L))
+    check(irem(1))
+    check(lrem(1L))
+  }
+}

test/files/run/t8601b.flags

@@ -0,0 +1 @@
+-optimize

test/files/run/t8601b.scala

@@ -0,0 +1,9 @@
+object Test {
+  def len(x: Array[String]): Unit = x.length
+
+  def check(x: => Any) = try { x; sys.error("failed to throw NPE!") } catch { case _: NullPointerException => }
+
+  def main(args: Array[String]) {
+    check(len(null))
+  }
+}