Neither gcc or icc support this. Split out from D79472. I want
to remove more, but it looks like icc does support some things
gcc doesn't and I need to double check our internal test suites.
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 D35205
llvm counterpart: D36369
Differential Revision: https://reviews.llvm.org/D36371
llvm-svn: 311643