Use IRVariable's in IR code generation and implement tuples.

test/cmdlineTests/yul_string_format_ascii/output.json

@@ -34,8 +34,8 @@ object \"C_10\" {
 
         }
 
-        function fun_f_9()  -> vloc__4 {
-            vloc__4 := convert_t_stringliteral_9f0adad0a59b05d2e04a1373342b10b9eb16c57c164c8a3bfcbf46dccee39a21_to_t_string_memory_ptr()
+        function fun_f_9()  -> vloc__4_mpos {
+            vloc__4_mpos := convert_t_stringliteral_9f0adad0a59b05d2e04a1373342b10b9eb16c57c164c8a3bfcbf46dccee39a21_to_t_string_memory_ptr()
             leave
 
         }
@@ -130,8 +130,8 @@ object \"C_10\" {
                 }
             }
 
-            function fun_f_9()  -> vloc__4 {
-                vloc__4 := convert_t_stringliteral_9f0adad0a59b05d2e04a1373342b10b9eb16c57c164c8a3bfcbf46dccee39a21_to_t_string_memory_ptr()
+            function fun_f_9()  -> vloc__4_mpos {
+                vloc__4_mpos := convert_t_stringliteral_9f0adad0a59b05d2e04a1373342b10b9eb16c57c164c8a3bfcbf46dccee39a21_to_t_string_memory_ptr()
                 leave
 
             }

test/cmdlineTests/yul_string_format_ascii_long/output.json

@@ -38,8 +38,8 @@ object \"C_10\" {
 
         }
 
-        function fun_f_9()  -> vloc__4 {
-            vloc__4 := convert_t_stringliteral_d6604f85ac07e2b33103a620b3d3d75b0473c7214912beded67b9b624d41c571_to_t_string_memory_ptr()
+        function fun_f_9()  -> vloc__4_mpos {
+            vloc__4_mpos := convert_t_stringliteral_d6604f85ac07e2b33103a620b3d3d75b0473c7214912beded67b9b624d41c571_to_t_string_memory_ptr()
             leave
 
         }
@@ -138,8 +138,8 @@ object \"C_10\" {
                 }
             }
 
-            function fun_f_9()  -> vloc__4 {
-                vloc__4 := convert_t_stringliteral_d6604f85ac07e2b33103a620b3d3d75b0473c7214912beded67b9b624d41c571_to_t_string_memory_ptr()
+            function fun_f_9()  -> vloc__4_mpos {
+                vloc__4_mpos := convert_t_stringliteral_d6604f85ac07e2b33103a620b3d3d75b0473c7214912beded67b9b624d41c571_to_t_string_memory_ptr()
                 leave
 
             }

test/libsolidity/semanticTests/viaYul/assign_tuple_from_function_call.sol

@@ -0,0 +1,16 @@
+contract C {
+    function f() public pure returns (uint, uint, uint) {
+        return (1, 2, 3);
+    }
+    function g() public pure returns (uint a, uint b, uint c) {
+        (c, b, a) = f();
+    }
+    function h() public pure returns (uint a) {
+        (,,a) = f();
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// g() -> 3, 2, 1
+// h() -> 3

test/libsolidity/semanticTests/viaYul/define_tuple_from_function_call.sol

@@ -0,0 +1,18 @@
+contract C {
+    function f() public pure returns (uint, uint, uint) {
+        return (1, 2, 3);
+    }
+    function g() public pure returns (uint x, uint y, uint z) {
+        (uint c, uint b, uint a) = f();
+        (x, y, z) = (a, b, c);
+    }
+    function h() public pure returns (uint) {
+        (,,uint a) = f();
+        return a;
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// g() -> 3, 2, 1
+// h() -> 3

test/libsolidity/semanticTests/viaYul/local_tuple_assignment.sol

@@ -0,0 +1,32 @@
+contract C {
+    uint public x = 17;
+    function f(uint a1, uint a2) public returns (uint r1, uint r2) {
+        (uint b1, uint b2) = (a1, a2);
+        (r1, x, r2) = (b1, b2, b2);
+    }
+    function g() public returns (uint a, uint b, uint c) {
+        uint256[3] memory m;
+        (m[0], m[1], m[2]) = (1, x, 3);
+        return (m[2], m[1], m[0]);
+    }
+    function h() public returns (uint a, uint b, uint c) {
+        uint256[3] memory m;
+        (m[0], m[1], , m[2], m[0]) = (1, x, 3, 4, 42);
+        return (m[2], m[1], m[0]);
+    }
+    function i() public returns (uint a, uint b, uint c, uint d) {
+        (a) = 42;
+        (((((b))))) = 23;
+        c = (((17)));
+        (((d))) = (13);
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// x() -> 17
+// f(uint256,uint256): 23, 42 -> 23, 42
+// x() -> 42
+// g() -> 3, 42, 1
+// h() -> 4, 42, 1
+// i() -> 42, 23, 17, 13

test/libsolidity/semanticTests/viaYul/local_variable_without_init.sol

@@ -0,0 +1,10 @@
+contract C {
+    function f() public pure returns (uint) {
+        uint x;
+        return x;
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// f() -> 0

test/libsolidity/semanticTests/viaYul/tuple_evaluation_order.sol

@@ -0,0 +1,14 @@
+contract C {
+    uint256 x;
+    uint256 y;
+    function set(uint256 v) public returns (uint256) { x = v; return v; }
+    function f() public returns (uint256, uint256) {
+       (y, y, y) = (set(1), set(2), set(3));
+       assert(y == 1 && x == 3);
+       return (x, y);
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// f() -> 3, 1