SI-8531 Better space efficiency for patmat analysis

By adding logging to `clause`, I found that the majority of
calls provide 0 or 1 elements. In SI-7020 / 69557da55, we changed this
method to use a `LinkedHashSet` to have deterministic results
for clauses with more elements. But I suspect that this
contributes to higher memory usage from the pattern matcher.

The enclosed test case, carefully whittled down by @oxbowlakes,
used to consume an inordinate amount of memory and time.
After this patch, it is back to 2.10.4 performance.

I have run `neg/t7020.scala` in a loop and it still is deterministic.

1 files changed, 24 insertions, 0 deletions

diff --git a/test/files/pos/t8531/Test.scala b/test/files/pos/t8531/Test.scala
new file mode 100644
index 0000000000..59861435a6
--- /dev/null
+++ b/test/files/pos/t8531/Test.scala
@@ -0,0 +1,24 @@
+package test
+
+// takes > 50s and > 800M heap to compile under 2.11.0
+import foobar._
+class `SI-8531` {
+    //https://issues.scala-lang.org/browse/SI-8531
+
+  import MyEnum._
+  def foo(e1: MyEnum, e2: MyEnum) = (e1, e2) match {
+    case (A1, x) => "a1"
+    case (x, A1) => "a1"
+    case (A2, x) => "a2"
+    case (x, A2) => "a2"
+    case (A3, x) => "a3"
+    case (x, A3) => "a3"
+    case (A4, x) => "a4"
+    case (x, A4) => "a4"
+    case (A5, x) => "a5"
+    case (x, A5) => "a5"
+    case (A6, x) => "a6"
+    case (x, A6) => "a6"
+    case (a, b)  => "ab"
+  }
+}