MS Intrin.h: implement __cpuidex and simplify __cpuid

The two identical implementations of __cpuid for X86 / X86_64 were
leftovers from my first iteration on the patch that implemented it.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@200568 91177308-0d34-0410-b5e6-96231b3b80d8