Allow exponentials with signed base and unsigned power.

test/libsolidity/semanticTests/exponentiation/signed_base.sol

@@ -0,0 +1,14 @@
+contract test {
+    function f() public pure returns (int, int) {
+        int32 x = -3;
+        uint8 y1;
+        uint8 y2;
+        assembly {
+            y1 := 0x102
+            y2 := 0x103
+        }
+        return (x**y1, x**y2);
+    }
+}
+// ----
+// f() -> 9, -27

test/libsolidity/semanticTests/exponentiation/small_exp.sol

@@ -0,0 +1,13 @@
+contract test {
+    function f() public pure returns (uint) {
+        uint32 x;
+        uint8 y;
+        assembly {
+            x := 0xfffffffffe
+            y := 0x102
+        }
+        return x**y;
+    }
+}
+// ----
+// f() -> 4