Optimize exp when base is -1

test/libsolidity/gasTests/exp.sol

@@ -0,0 +1,29 @@
+pragma experimental ABIEncoderV2;
+
+contract C {
+	function exp_neg_one(uint exponent) public returns(int) {
+		return (-1)**exponent;
+	}
+	function exp_two(uint exponent) public returns(uint) {
+		return 2**exponent;
+	}
+	function exp_zero(uint exponent) public returns(uint) {
+		return 0**exponent;
+	}
+	function exp_one(uint exponent) public returns(uint) {
+		return 1**exponent;
+	}
+}
+// ====
+// optimize: false
+// optimize-yul: false
+// ----
+// creation:
+//   codeDepositCost: 119800
+//   executionCost: 165
+//   totalCost: 119965
+// external:
+//   exp_neg_one(uint256): 2259
+//   exp_one(uint256): infinite
+//   exp_two(uint256): infinite
+//   exp_zero(uint256): infinite

test/libsolidity/gasTests/exp_optimized.sol

@@ -0,0 +1,29 @@
+pragma experimental ABIEncoderV2;
+
+contract C {
+	function exp_neg_one(uint exponent) public returns(int) {
+		return (-1)**exponent;
+	}
+	function exp_two(uint exponent) public returns(uint) {
+		return 2**exponent;
+	}
+	function exp_zero(uint exponent) public returns(uint) {
+		return 0**exponent;
+	}
+	function exp_one(uint exponent) public returns(uint) {
+		return 1**exponent;
+	}
+}
+// ====
+// optimize: true
+// optimize-yul: true
+// ----
+// creation:
+//   codeDepositCost: 53200
+//   executionCost: 105
+//   totalCost: 53305
+// external:
+//   exp_neg_one(uint256): 1962
+//   exp_one(uint256): 1915
+//   exp_two(uint256): 1893
+//   exp_zero(uint256): 1937