[X86] Change the name string for the newly add DF flag register to 'dirflag'...

[X86] Change the name string for the newly add DF flag register to 'dirflag' to match the clobber name supported by clang for MS inline assembly.

This should fix the failure found by Chromium reported here https://bugs.chromium.org/p/chromium/issues/detail?id=831158

The test case will be added in clang.

llvm-svn: 329734