[CMAKE] fix LLVM_OPTIMIZED_TABLEGEN for Visual Studio

At the moment optimized tablegen is generated by LLVM_USE_HOST_TOOLS variable that is not set for Visual Sudio since LLVM_ENABLE_ASSERTIONS depends on CMAKE_BUILD_TYPE value that is not equal to "DEBUG" in case of Visual Studio soltion generation.

Modified to do not depend on LLVM_ENABLE_ASSERTIONS value in VS and Xcode cases

Reviewers: beanz

Subscribers: RKSimon, llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D27135

llvm-svn: 288042