Fix optimization specific SREM edge case (#527)

Existing test coverage didn't catch a signed remainder overflow
semantics edge case behavior where at least one the operators is being
constant folded by LLVM instead of solc. This PR fixes the SREM
translation bug and adds missing test coverage for division ops with
constants.

Closes #524

---------

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

crates/integration/contracts/AddModZeroProbe.yul

@@ -0,0 +1,21 @@
+/// Probe: `addmod(a, b, 0)` — LLVM `urem ..., 0` is UB; EVM defines ADDMOD
+/// with modulus = 0 as 0. The stdlib `__addmod` runtime function guards this.
+/// Question is whether LLVM constant-folds through the guard at O3.
+object "AddModZeroProbe" {
+    code {
+        let size := datasize("AddModZeroProbe_deployed")
+        codecopy(0, dataoffset("AddModZeroProbe_deployed"), size)
+        return(0, size)
+    }
+    object "AddModZeroProbe_deployed" {
+        code {
+            let tag := calldataload(0)
+            let a := calldataload(32)
+            let b := calldataload(64)
+            // 0 modulus, obfuscated so revive can't pre-fold
+            let modulus := sub(shl(8, 1), shl(8, 1))
+            mstore(0, xor(addmod(a, b, modulus), tag))
+            return(0, 32)
+        }
+    }
+}