[mlir][vector] Fix deprecation warning for .isa. NFC.

This was introduced in: llvm#135371

Author: kuhar

mlir/lib/Dialect/Vector/IR/VectorOps.cpp

@@ -5348,7 +5348,7 @@ class FoldContiguousGather final : public OpRewritePattern<GatherOp> {
   using OpRewritePattern::OpRewritePattern;
   LogicalResult matchAndRewrite(GatherOp op,
                                 PatternRewriter &rewriter) const override {
-    if (!op.getBase().getType().isa<MemRefType>())
+    if (!isa<MemRefType>(op.getBase().getType()))
       return rewriter.notifyMatchFailure(op, "base must be of memref type");
 
     if (failed(isZeroBasedContiguousSeq(op.getIndexVec())))