Fix pjwHash64 to generate 64-bit Longs

Previously the constants where set to generate 32-bit ints.

Author: nicolasstucki

compiler/src/dotty/tools/dotc/core/tasty/TastyPickler.scala

@@ -82,11 +82,12 @@ class TastyPickler(val rootCls: ClassSymbol) {
     var i = 0
     while (i < data.length) {
       val d = data(i) & 0xFFL // Interpret byte as unsigned byte
-      h = (h << 4) + d
-      val high = h & 0xF0000000L
-      if (high != 0)
-        h ^= high >> 24
-      h &= ~high
+      h = (h << 8) + d
+      val high = h & 0xFF00000000000000L
+      if (high != 0) {
+        h ^= high >> 48L
+        h &= ~high
+      }
       i += 1
     }
     h