[multi-gpu] Phase 1: fix negative-rank verifier check

The previous commit (888bcaa) added a `>= 0` verifier on src_rank /
dst_rank, but used `getSrcRank()` / `getDstRank()` — those return
`std::optional<uint64_t>` (a TableGen quirk for `OptionalAttr<I64Attr>`),
so `*sr < 0` on the unsigned value is always false and the check never
fired. The two new verifier-negative tests in air_memcpy_invalid.mlir
silently regressed.

Switch to the typed `getSrcRankAttr()` / `getDstRankAttr()` accessors
which return `IntegerAttr`, then call `.getInt()` for a real `int64_t`.
The check now fires on negative values; both negative-rank tests pass
under `lit -sv ../../mlir/test/Dialect/AIR` (21/21).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: erwei-xilinx

mlir/lib/Dialect/AIR/IR/AIRDialect.cpp

@@ -2823,13 +2823,20 @@ LogicalResult air::DmaMemcpyNdOp::verify() {
       return emitOpError("src_rank/dst_rank attributes require an enclosing "
                          "air.rank scope");
 
-    // Rank indices are non-negative.
-    if (auto sr = getSrcRank())
-      if (*sr < 0)
-        return emitOpError() << "src_rank must be >= 0, got " << *sr;
-    if (auto dr = getDstRank())
-      if (*dr < 0)
-        return emitOpError() << "dst_rank must be >= 0, got " << *dr;
+    // Rank indices are non-negative. Use the typed *Attr accessor instead
+    // of the generated getSrcRank()/getDstRank() (those return uint64_t
+    // for OptionalAttr<I64Attr>, so a comparison against 0 is meaningless
+    // for negative values stored as i64).
+    if (auto srAttr = getSrcRankAttr()) {
+      int64_t sr = srAttr.getInt();
+      if (sr < 0)
+        return emitOpError() << "src_rank must be >= 0, got " << sr;
+    }
+    if (auto drAttr = getDstRankAttr()) {
+      int64_t dr = drAttr.getInt();
+      if (dr < 0)
+        return emitOpError() << "dst_rank must be >= 0, got " << dr;
+    }
 
     auto requireSymmetricAlloc = [&](Value v, StringRef side) -> LogicalResult {
       auto alloc = v.getDefiningOp<memref::AllocOp>();