Use an *unsigned* right shift

Found by comparing testcases with the C version, and by then comparing the
source code; both C versions use an unsigned right shift (or rather a right
shift on unsigned integers).

* Background:

  Right-shifting a signed number copies the sign bit into all the new "empty
  bits"; C uses the integer sign, while Java (and Scala) use a separate
  "unsigned right shift" operator.

Author: Blaisorblade

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

@@ -13,7 +13,7 @@ object TastyHash {
       val d = data(i) & 0xFFL // Interpret byte as unsigned byte
       h = (h << 8) + d
       val high = h & 0xFF00000000000000L
-      h ^= high >> 48L
+      h ^= high >>> 48L
       h &= ~high
       i += 1
     }

tests/run-with-compiler/tasty-hash.scala

@@ -13,7 +13,7 @@ object Test {
     testHash(0X101010101L, Array(1, 1, 1, 1, 1))
     testHash(0X202020202L, Array(2, 2, 2, 2, 2))
     testHash(0X1010101L, Array.fill(1024)(1))
-    testHash(0X55aa01fe55ab54ffL, Array.tabulate(1024)(_.toByte))
+    testHash(0xaafefeaaab54ffL, Array.tabulate(1024)(_.toByte))
     testHash(0x34545c16020230L, "abcdefghijklmnopqrstuvwxyz1234567890".getBytes)
   }