[MLIR] Fix BubbleDownVectorBitCastForExtract crash on non-static index (llvm#116518)

Previously the patch was not expecting to handle non-static index, when
the index is a non constant value it will crash.
    
This patch is to make sure it return gracefully instead of crashing.

Author: lialan

mlir/lib/Dialect/Vector/Transforms/VectorTransforms.cpp

@@ -596,12 +596,11 @@ struct BubbleDownVectorBitCastForExtract
     unsigned expandRatio =
         castDstType.getNumElements() / castSrcType.getNumElements();
 
-    auto getFirstIntValue = [](ArrayRef<OpFoldResult> values) -> uint64_t {
-      assert(values[0].is<Attribute>() && "Unexpected non-constant index");
-      return cast<IntegerAttr>(values[0].get<Attribute>()).getInt();
-    };
-
-    uint64_t index = getFirstIntValue(extractOp.getMixedPosition());
+    // Get the first element of the mixed position as integer.
+    auto mixedPos = extractOp.getMixedPosition();
+    if (mixedPos.size() > 0 && !mixedPos[0].is<Attribute>())
+      return failure();
+    uint64_t index = cast<IntegerAttr>(mixedPos[0].get<Attribute>()).getInt();
 
     // Get the single scalar (as a vector) in the source value that packs the
     // desired scalar. E.g. extract vector<1xf32> from vector<4xf32>

mlir/test/Dialect/Vector/vector-transforms.mlir

@@ -433,3 +433,16 @@ func.func @vec_0D(%arg0: vector<f32>) -> vector<i32> {
   %0 = vector.bitcast %arg0 : vector<f32> to vector<i32>
   return %0 : vector<i32>
 }
+
+// Make sure not crash on dynamic index `vector.extract`:
+func.func @vector_extract_dynamic_index(%arg0 : vector<4xi32>, %index : index) -> i16 {
+  %0 = vector.bitcast %arg0 : vector<4xi32> to vector<8xi16>
+  %1 = vector.extract %0[%index] : i16 from vector<8xi16>
+  return %1 : i16
+}
+
+// CHECK-LABEL: func.func @vector_extract_dynamic_index
+// CHECK-SAME: (%[[VEC:.+]]: vector<4xi32>, %[[IDX:.+]]: index) -> i16 {
+// CHECK: %[[BC:.+]] = vector.bitcast %[[VEC]] : vector<4xi32> to vector<8xi16>
+// CHECK: %[[EXTRACT:.+]] = vector.extract %[[BC]][%[[IDX]]] : i16 from vector<8xi16>
+// CHECK: return %[[EXTRACT]]