Implement signed subtraction for sol->yul code generation.

test/libsolidity/semanticTests/viaYul/detect_sub_overflow.sol

@@ -0,0 +1,17 @@
+contract C {
+    function f(uint a, uint b) public pure returns (uint x) {
+        x = a - b;
+    }
+    function g(uint8 a, uint8 b) public pure returns (uint8 x) {
+        x = a - b;
+    }
+}
+// ====
+// compileViaYul: true
+// ----
+// f(uint256,uint256): 6, 5 -> 1
+// f(uint256,uint256): 6, 6 -> 0
+// f(uint256,uint256): 5, 6 -> FAILURE
+// g(uint8,uint8): 6, 5 -> 1
+// g(uint8,uint8): 6, 6 -> 0
+// g(uint8,uint8): 5, 6 -> FAILURE

test/libsolidity/semanticTests/viaYul/detect_sub_overflow_signed.sol

@@ -0,0 +1,42 @@
+contract C {
+    function f(int a, int b) public pure returns (int x) {
+        x = a - b;
+    }
+    function g(int8 a, int8 b) public pure returns (int8 x) {
+        x = a - b;
+    }
+}
+// ====
+// compileViaYul: true
+// ----
+// f(int256,int256): 5, 6 -> -1
+// f(int256,int256): -2, 1 -> -3
+// f(int256,int256): -2, 2 -> -4
+// f(int256,int256): 2, -2 -> 4
+// f(int256,int256): 2, 2 -> 0
+// f(int256,int256): -5, -6 -> 1
+// f(int256,int256): 0x7FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF0, -15 -> 0x7FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF
+// f(int256,int256): 0x7FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF0, -16 -> FAILURE
+// f(int256,int256): 0x7FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF, -1 -> FAILURE
+// f(int256,int256): 15, 0x8000000000000000000000000000000000000000000000000000000000000010 -> 0x7FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF
+// f(int256,int256): 16, 0x8000000000000000000000000000000000000000000000000000000000000010 -> FAILURE
+// f(int256,int256): 1, 0x8000000000000000000000000000000000000000000000000000000000000000 -> FAILURE
+// f(int256,int256): -1, 0x7FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF -> 0x8000000000000000000000000000000000000000000000000000000000000000
+// f(int256,int256): -2, 0x7FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF -> FAILURE
+// f(int256,int256): 0x8000000000000000000000000000000000000000000000000000000000000001, 1 -> 0x8000000000000000000000000000000000000000000000000000000000000000
+// f(int256,int256): 0x8000000000000000000000000000000000000000000000000000000000000001, 2 -> FAILURE
+// f(int256,int256): 0x8000000000000000000000000000000000000000000000000000000000000000, 1 -> FAILURE
+// g(int8,int8): 5, 6 -> -1
+// g(int8,int8): -2, 1 -> -3
+// g(int8,int8): -2, 2 -> -4
+// g(int8,int8): 2, -2 -> 4
+// g(int8,int8): 2, 2 -> 0
+// g(int8,int8): -5, -6 -> 1
+// g(int8,int8): 126, -1 -> 127
+// g(int8,int8): 1, -126 -> 127
+// g(int8,int8): 127, -1 -> FAILURE
+// g(int8,int8): 1, -127 -> FAILURE
+// g(int8,int8): -127, 1 -> -128
+// g(int8,int8): -1, 127 -> -128
+// g(int8,int8): -127, 2 -> FAILURE
+// g(int8,int8): -2, 127 -> FAILURE