ms inline asm: Don't add x86 segment registers to the clobber list.

Clang tries to check the clobber list but doesn't list segment registers in its
x86 register list. This fixes PR20343.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213303 91177308-0d34-0410-b5e6-96231b3b80d8