BCodeOpt: test constant folding including nullness propagation

A test is added to verify branches are removed when their outcome
can be determined at compile time based on constant-folding and
nullness propagation.

The test is the BCodeOpt counterpart to the jvm test introduced in
commit 69109c0

test/files/jvm/bcode-constant-optimization/CF_1.flags

@@ -0,0 +1 @@
+-neo:o1 -Ynooptimize

test/files/jvm/bcode-constant-optimization/CF_1.scala

@@ -0,0 +1,11 @@
+
+class CF_1 {
+  def foo() {
+    // constant optimization should eliminate all branches
+    val i = 1
+    val x = if (i != 1) null else "good"
+    val y = if (x == null) "good" else x + ""
+    println(y)
+  }
+}
+

test/files/jvm/bcode-constant-optimization/CF_Test.scala

@@ -0,0 +1,27 @@
+
+import scala.tools.partest.BytecodeTest
+import scala.tools.asm
+import asm.tree.InsnList
+import scala.collection.JavaConverters._
+
+object Test extends BytecodeTest {
+  val comparisons = Set(asm.Opcodes.IF_ACMPEQ, asm.Opcodes.IF_ACMPNE, asm.Opcodes.IF_ICMPEQ, asm.Opcodes.IF_ICMPGE, asm.Opcodes.IF_ICMPGT, asm.Opcodes.IF_ICMPLE,
+    asm.Opcodes.IF_ICMPLT, asm.Opcodes.IF_ICMPNE, asm.Opcodes.IFEQ, asm.Opcodes.IFGE, asm.Opcodes.IFGT, asm.Opcodes.IFLE, asm.Opcodes.IFLT, 
+    asm.Opcodes.IFNE, asm.Opcodes.IFNONNULL, asm.Opcodes.IFNULL)
+
+  def show: Unit = {
+    val classNode = loadClassNode("CF_1")
+    val methodNode = getMethod(classNode, "foo")
+    // after optimization there should be no comparisons left
+    val expected = 0
+
+    val got = countComparisons(methodNode.instructions)
+    assert(got == expected, s"expected $expected but got $got comparisons")
+  }
+
+  def countComparisons(insnList: InsnList): Int = {
+    def isComparison(node: asm.tree.AbstractInsnNode): Boolean =
+      (comparisons contains node.getOpcode)
+    insnList.iterator.asScala count isComparison
+  }
+}