Revert "[x86][inline-asm][clang] accept 'v' constraint"

This reverts commit r283716.

Breaks buildbot:
http://lab.llvm.org:8080/green/job/clang-stage2-configure-Rlto_check/9155/testReport/junit/Clang/CodeGen/x86_inline_asm_v_constraint_c/

llvm-svn: 283743