[mlir] Fix DenseElementsAttr::mapValues(i1, splat).

Splat of bool is encoded as a byte with all-ones in it [1]. Without this
change, this piece of code:

    auto xs = builder.getI32TensorAttr({42, 42, 42, 42});
    auto xs2 = xs.mapValues(builder.getI1Type(), [](const llvm::APInt &x) {
      return x.isZero() ? llvm::APInt::getZero(1) : llvm::APInt::getAllOnes(1);
    });
    xs2.dump();

Prints:

    dense<[true, false, false, false]> : tensor<4xi1>

Because only the first bit is set. This applies to both
DenseIntElementsAttr::mapValues() and DenseFPElementsAttr::mapValues().

[1]: https://github.com/llvm/llvm-project/blob/e877b42e2c70813352c1963ea33e992f481d5cba/mlir/lib/IR/BuiltinAttributes.cpp#L984

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D132767