Fix exponentiation bug

test/libsolidity/syntaxTests/literalOperations/exponent.sol

@@ -0,0 +1,10 @@
+contract C {
+    function g() public pure {
+        int a;
+        a ** 1E1233;
+        a ** (1/2);
+    }
+}
+// ----
+// TypeError: (67-78): Operator ** not compatible with types int256 and int_const 1000...(1226 digits omitted)...0000. Exponent too large.
+// TypeError: (88-98): Operator ** not compatible with types int256 and rational_const 1 / 2. Exponent is fractional.

test/libsolidity/syntaxTests/literalOperations/exponent_fine.sol

@@ -0,0 +1,7 @@
+contract C {
+    function f() public pure {
+        uint a;
+        a = a ** 1E5;
+        a = 0 ** 1E1233;
+    }
+}