[Mosaic TPU] Fold sublane offset to indices when storing to untiled ref.

This optimization avoids unnecessary retiling when storing to untiled ref but adds at most one extra store op for sublane offset (since sublane offset is limieted to < VregSlice[0]).

PiperOrigin-RevId: 698896373

Author: bythew3i

jaxlib/mosaic/dialect/tpu/transforms/infer_vector_layout.cc

@@ -1640,14 +1640,14 @@ class VectorLayoutInferer {
         // Since it is untiled, we can store to any arbitrary address which
         // means the sublane offset can be any value and we can fold it to
         // 2nd minor index.
-        // TODO(jevinjiang): We can fold the sublane offset into the 2nd minor
-        // index. But we need to handle negative index in lower-to-llo. For
-        // now, we just force the sublane offset to be 0.
+        auto prev_store_layout = getLayout(op.getValueToStore());
+        TPU_CHECK_OP(prev_store_layout.has_value(), "missing vector layout");
+        offsets[0] = prev_store_layout->offsets()[0].value_or(0);
         if (offsets[1].value_or(0) >= tiling[1]) {
           offsets[1] = 0;
         }
-        store_layout = VectorLayout(bitwidth, {0, offsets[1]},
-                                    nativeTiling(bitwidth), ImplicitDim::kNone);
+        store_layout = VectorLayout(bitwidth, offsets, nativeTiling(bitwidth),
+                                    ImplicitDim::kNone);
       } else {
         store_layout = VectorLayout(bitwidth, offsets, {tiling[0], tiling[1]},
                                     ImplicitDim::kNone);