llvm-context: fix off-by-one in SDIV overflow semantics (#398)

- Fix the wrong predicate (the negative side in 2s complement has one
more bit)
- Increase test coverage

Signed-off-by: xermicus <cyrill@parity.io>

crates/integration/codesize.json

@@ -1,7 +1,7 @@
 {
   "Baseline": 914,
   "Computation": 2295,
-  "DivisionArithmetics": 14510,
+  "DivisionArithmetics": 14496,
   "ERC20": 17482,
   "Events": 1674,
   "FibonacciIterative": 1490,

crates/integration/src/tests.rs

@@ -169,6 +169,8 @@ fn signed_division() {
         (minus_five, two),
         (I256::MINUS_ONE, I256::MIN),
         (one, I256::ZERO),
+        (I256::MIN, I256::MINUS_ONE),
+        (I256::MIN + I256::ONE, I256::MINUS_ONE),
     ] {
         actions.push(Call {
             origin: TestAddress::Alice,