[Yul] Support conditionals

test/libsolidity/semanticTests/viaYul/conditional/conditional_multiple.sol

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+contract A {
+	function f() public pure returns (uint) {
+		uint x = 3 < 0 ? 2 > 1 ? 2 : 1 : 7 > 2 ? 7 : 6;
+		return x;
+	}
+}
+// ====
+// compileViaYul: also
+// ----
+// f() -> 7

test/libsolidity/semanticTests/viaYul/conditional/conditional_true_false_literal.sol

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+contract A {
+	function f() public pure returns (uint) {
+		uint x = true ? 1 : 0;
+		uint y = false ? 0 : 1;
+		return x + y;
+	}
+}
+// ====
+// compileViaYul: also
+// ----
+// f() -> 2

test/libsolidity/semanticTests/viaYul/conditional/conditional_tuple.sol

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+contract A {
+	function f(bool cond) public pure returns (uint, uint) {
+		(uint a, uint b) = cond ? (1, 2) : (3, 4);
+		return (a, b);
+	}
+}
+// ====
+// compileViaYul: also
+// ----
+// f(bool): true -> 1, 2
+// f(bool): false -> 3, 4

test/libsolidity/semanticTests/viaYul/conditional/conditional_with_assignment.sol

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+contract A {
+    function f() public pure returns (uint, uint, uint, uint) {
+		uint y1 = 1;
+		uint y2 = 1;
+		uint x = 3 < 0 ? y1 = 3 : 6;
+		uint z = 3 < 10 ? y2 = 5 : 6;
+		return (x, y1, y2, z);
+	}
+}
+// ====
+// compileViaYul: also
+// ----
+// f() -> 6, 1, 5, 5

test/libsolidity/semanticTests/viaYul/conditional/conditional_with_variables.sol

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+contract A {
+	function f() public pure returns (uint, uint, uint, uint) {
+		uint x = 3;
+		uint y = 1;
+		uint z = (x > y) ? x : y;
+		uint w = x < y ? x : y;
+		return (x, y, z, w);
+	}
+}
+// ====
+// compileViaYul: also
+// ----
+// f() -> 3, 1, 3, 1