X86: cpuid and xgetbv write to 32-bit registers, not 64-bit

This fixes an issue where MS inline assembly containing xgetbv wouldn't
be marked as clobbering EAX:EDX. Test for that forthcoming on the Clang
side.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217173 91177308-0d34-0410-b5e6-96231b3b80d8