Fixes u256 overflow in logical shift optimization rule and adds tests.

docs/bugs.json

@@ -1,4 +1,16 @@
 [
+    {
+        "name": "DoubleShiftSizeOverflow",
+        "summary": "Double bitwise shifts by large constants whose sum overflows 256 bits can result in unexpected values.",
+        "description": "Nested logical shift operations whose total shift size is 2**256 or more are incorrectly optimized. This only applies to shifts by numbers of bits that are compile-time constant expressions.",
+        "introduced": "0.5.5",
+        "fixed": "0.5.6",
+        "severity": "low",
+        "conditions": {
+            "optimizer": true,
+            "evmVersion": ">=constantinople"
+        }
+    },
     {
         "name": "ExpExponentCleanup",
         "summary": "Using the ** operator with an exponent of type shorter than 256 bits can result in unexpected values.",