[LLVM][x86][Inline Asm] support for GCC style inline asm - Y<x> constraints

This patch is intended to enable the use of basic double letter constraints used in GCC extended inline asm {Yi Y2 Yz Y0 Ym Yt}.
Supersedes D35204
Clang counterpart: D36371

Differential Revision: https://reviews.llvm.org/D36369

llvm-svn: 311644