[X86] Print %st(0) as %st to match what gcc inline asm uses as the clobber name to...

[X86] Print %st(0) as %st to match what gcc inline asm uses as the clobber name to make MS inline asm work correctly

Summary:
When calculating clobbers for MS style inline assembly we fail if the asm clobbers stack top because we print st(0) and try to pass it through the gcc register name check. This was found with when I attempted to make a emms/femms clobber all ST registers. If you use emms/femms in MS inline asm we would try to use st(0) as the clobber name but clang would think that wasn't a valid clobber name.

This also matches what objdump disassembly prints. It's also what is printed by gcc -S.

Reviewers: RKSimon, rnk, efriedma, spatel, andreadb, lebedev.ri

Reviewed By: rnk

Subscribers: eraman, gbedwell, lebedev.ri, llvm-commits

Differential Revision: https://reviews.llvm.org/D57621

llvm-svn: 352985