Prevent potential NOREX bug.

A GR8_NOREX virtual register is created when extrating a sub_8bit_hi
sub-register:

  %vreg2<def> = COPY %vreg1:sub_8bit_hi; GR8_NOREX:%vreg2 %GR64_ABCD:%vreg1
  TEST8ri_NOREX %vreg2, 1, %EFLAGS<imp-def>; GR8_NOREX:%vreg2

If such a live range is ever split, its register class must not be
inflated to GR8.  The sub-register copy can only target GR8_NOREX.

I dont have a test case for this theoretical bug.

llvm-svn: 141500