[X86] Permit reading of the FLAGS register without it being previously defined

We modeled the RDFLAGS{32,64} operations as "using" {E,R}FLAGS.
While technically correct, this is not be desirable for folks who want
to examine aspects of the FLAGS register which are not related to
computation like whether or not CPUID is a valid instruction.

Differential Revision: http://reviews.llvm.org/D17782

llvm-svn: 262465