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/SdivBothConst.yul

@@ -0,0 +1,42 @@
+/// Both-constant SDIV variants. Selector index is read from calldata[0..32]
+/// and dispatched to a branch with two literal operands. Compiled directly via
+/// revive's Yul-to-LLVM path (bypassing solc's Yul optimizer) so the literals
+/// reach LLVM intact and exercise revive's "two literal operands" codegen path.
+///
+/// Case 11 — sdiv(INT_MIN, -1) — is the signed-overflow edge case that is UB
+/// in LLVM `sdiv` and must be guarded in revive's codegen.
+object "SdivBothConst" {
+    code {
+        let size := datasize("SdivBothConst_deployed")
+        codecopy(0, dataoffset("SdivBothConst_deployed"), size)
+        return(0, size)
+    }
+    object "SdivBothConst_deployed" {
+        code {
+            let which := calldataload(0)
+            // `tag` is XORed into the result so that any poison/undef LLVM
+            // produces from a UB-triggering const-fold (e.g. sdiv(INT_MIN,-1))
+            // propagates and diverges from EVM, instead of coincidentally
+            // returning 0 and masking the bug.
+            let tag := calldataload(32)
+            let r := 0
+            switch which
+            case 0  { r := sdiv(5, 5) }
+            case 1  { r := sdiv(5, 1) }
+            case 2  { r := sdiv(0, 0x7fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff) }
+            case 3  { r := sdiv(0, 0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff) }
+            case 4  { r := sdiv(5, 2) }
+            case 5  { r := sdiv(5, 0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff) }
+            case 6  { r := sdiv(0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff, 0xfffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffe) }
+            case 7  { r := sdiv(0xfffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffb, 0xfffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffb) }
+            case 8  { r := sdiv(0xfffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffb, 2) }
+            case 9  { r := sdiv(0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff, 0x8000000000000000000000000000000000000000000000000000000000000000) }
+            case 10 { r := sdiv(1, 0) }
+            case 11 { r := sdiv(0x8000000000000000000000000000000000000000000000000000000000000000, 0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff) }
+            case 12 { r := sdiv(0x8000000000000000000000000000000000000000000000000000000000000001, 0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff) }
+            default { revert(0, 0) }
+            mstore(0, xor(r, tag))
+            return(0, 32)
+        }
+    }
+}