[mlir] Fix for MSVC bool splat issue encountered.

When building MLIR using bazel on windows with MSVC2019, bool splats
were being created incorrectly:

```
dense<[true,true,true,true]> : tensor<4xi1>
-(parse with mlir-opt)-> dense<[true, false, false, false]> : tensor<4xi1>
```

Appears that a Windows bazel build produces a corrupt DenseIntOrFPElementsAttr.
Unable to repro using MSVC and cmake.

Issue first discovered here:
https://github.com/google/jax/issues/16394

Added test point for reproduction:

```
$ bazel test @llvm-project//mlir/unittests:ir_tests --test_arg=--gtest_filter=DenseSplatTest.BoolSplatSmall
```

Differential Revision: https://reviews.llvm.org/D155745