Set REG0_USED on X86 Set8R instruction

Since this instruction only affects the low byte of the register, it is
preceded by an XOR to zero the upper 3 bytes.  Set8R isn't marked as
using operand 0 as an input.  In practice, this works for now, as the
Xor sets the CC and Set8R uses the CC (although not that of the Xor, but
of a Cmp generally).

This just marks REG0 as using the previous contents of the register, as
it is modifying only 1 byte of 4.

Change-Id: I7a69cbdb06979da5d5d2ae17fabd7c22c5a17701
Signed-off-by: Mark Mendell <mark.p.mendell@intel.com>