amdgcn: Fix VGPR max count

The metadata for RDNA3 kernels allocates VGPRs in blocks of 12, which means the
maximum usable number of registers is 252.  This patch prevents the compiler
from exceeding this artifical limit.

gcc/ChangeLog:

	* config/gcn/gcn.cc (gcn_conditional_register_usage): Fix registers
	remaining after maximum allocation using TARGET_VGPR_GRANULARITY.