Add dependency on LLVM's not tool (#1829)

Was working in a corrupt build area and got a "command `not` not found"
error. CMake somehow works without it normally, but still better to add
it for correctness.

References:

- [StableHLO test using
`not`](https://github.com/openxla/stablehlo/blob/654c202babbab3f4bd50cdd276de02dc17ecc358/stablehlo/tests/vhlo_to_version_invalid.mlir#L4)
- [StableHLO bazel dependency on
`not`](https://github.com/openxla/stablehlo/blob/654c202babbab3f4bd50cdd276de02dc17ecc358/stablehlo/tests/BUILD.bazel#L150)
- [MHLO
example](https://github.com/openxla/xla/blob/fb67091154574430a8fd17b7eae7ceadd79f7139/xla/mlir_hlo/tests/CMakeLists.txt#L24)
using not dependency in cmake

stablehlo/tests/CMakeLists.txt

@@ -22,7 +22,7 @@ configure_lit_site_cfg(
 add_lit_testsuite(check-stablehlo-tests "Running the tests/ suite"
   ${CMAKE_CURRENT_BINARY_DIR}
   DEPENDS
-  FileCheck
+  FileCheck not
   stablehlo-opt
   stablehlo-translate
 )