amdgcn: silence warning

FIRST_SGPR_REG is register zero so the compiler always claims this comparison
is redundant.  It's right, of course, but I'd have preferred to keep the
comparison for completeness.  Probably the "correct" solution is to use an enum
for these values.

gcc/ChangeLog:

	* config/gcn/gcn.h (SGPR_REGNO_P): Silence warning.