[MLIR][GPU] Disallow llvm tanh intrinsics when lowering to NVVM/ROCm.

Summary:
The lowering to NVVM and ROCm handles tanh operations differently by
mapping them to NVVM/ROCm specific intrinsics. This conflicts with
the lowering to LLVM, which uses the default llvm intrinsic. This change
declares the LLVM intrinsics to be illegal, hence disallowing the
correspondign rewrite.

Differential Revision: https://reviews.llvm.org/D74389