SI-6514 Avoid spurious dead code warnings

`deadCode.expr` stores the method symbol most recently encountered
in `handleMonomorphicCall`, and uses this to avoid warnings
for arguments to label jumps and `Object#synchronized` (which
sneakily acts by-name without advertising the fact in its type.)

But this scheme was insufficient if the argument itself contains
another method call, such as `matchEnd(throw e(""))`.

This commit changes the single slot to a stack, and also
grants exemption to `LabelDef` trees. They were incorrectly
flagged in the enclosed test case after I made the the first change.

1 files changed, 11 insertions, 0 deletions

diff --git a/test/files/pos/t6514.scala b/test/files/pos/t6514.scala
new file mode 100644
index 0000000000..7c58605d39
--- /dev/null
+++ b/test/files/pos/t6514.scala
@@ -0,0 +1,11 @@
+object Test {
+  def e(msg: String) = new Exception(msg)
+
+  // this code ain't dead.
+  def a(b: Boolean) = {
+    b match {
+      case true => throw e("true")
+      case false => throw e("false")
+    }
+  }
+}