[X86, inline-asm] Allow 256-bit wide operands for the 'x' constraints

The 'x' constraint is for "any SSE register", and GCC seems to include the
256-bit ymm registers in that concept.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@218073 91177308-0d34-0410-b5e6-96231b3b80d8