Fixes bug in byte optimization rule and adds tests.

docs/bugs.json

@@ -1,4 +1,15 @@
 [
+    {
+        "name": "IncorrectByteInstructionOptimization",
+	"summary": "The optimizer incorrectly handles byte opcodes whose second argument is 31 or a constant expression that evaluates to 31. This can result in unexpected values.",
+        "description": "The optimizer incorrectly handles byte opcodes that use the constant 31 as second argument. This can happen when performing index access on bytesNN types with a compile-time constant value (not index) of 31 or when using the byte opcode in inline assembly.",
+        "introduced": "0.5.5",
+        "fixed": "0.5.7",
+        "severity": "very low",
+        "conditions": {
+            "optimizer": true
+        }
+    },
     {
         "name": "DoubleShiftSizeOverflow",
         "summary": "Double bitwise shifts by large constants whose sum overflows 256 bits can result in unexpected values.",

docs/bugs_by_version.json

@@ -631,12 +631,15 @@
     },
     "0.5.5": {
         "bugs": [
+            "IncorrectByteInstructionOptimization",
             "DoubleShiftSizeOverflow"
         ],
         "released": "2019-03-05"
     },
     "0.5.6": {
-        "bugs": [],
+        "bugs": [
+            "IncorrectByteInstructionOptimization"
+        ],
         "released": "2019-03-13"
     }
 }