It seems MS headers have started using __readgsqword, and since it's
used in a header that doesn't include intrin.h, we can't implement it as
an inline function anymore.
That was already the case for __readfsdword, which Saleem added support
for in r220859. This patch reuses that codegen to implement all of
__read[fg]s{byte,word,dword,qword}.
Differential Revision: https://reviews.llvm.org/D31248
llvm-svn: 298538
Hans Wennborg