[llvm-exegesis] Fix compile error on VS.

llvm::BitVector::const_set_bits_iterator is not formally a
ForwardIterator. Using it as such results in compile time errors on some
compilers:

FAILED: unittests/tools/llvm-exegesis/X86/CMakeFiles/LLVMExegesisX86Tests.dir/RegisterAliasingTest.cpp.obj
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE\xutility(967): error C2794: 'iterator_category': is not a member of any direct or indirect base class of 'std::iterator_traits<_InIt>'
        with
        [
            _InIt=llvm::BitVector::const_set_bits_iterator
        ]

llvm-svn: 332697