amdgcn: Don't double-count AVGPRs

CDNA2 devices have VGPRs and AVGPRs combined into a single hardware register
file (they're seperate in CDNA1).  I originally thought they were counted
separately in the vgpr_count and agpr_count metadata fields, and therefore
mkoffload had to account for this when passing the values to libgomp.  However,
that wasn't the case, and this code should have been removed when I corrected
the calculations in gcn.cc.  Fixing the error now.

gcc/ChangeLog:

	* config/gcn/mkoffload.cc (isa_has_combined_avgprs): Delete.
	(process_asm): Don't count avgprs.