amdgcn: Allow V64DFmode min/max reductions

I don't know why these were disabled. There're no direct min/max DPP
instructions for this mode, but the "use moves" strategy works fine.

gcc/ChangeLog:

	* config/gcn/gcn.c (gcn_expand_reduc_scalar): Use move instructions
	for V64DFmode min/max reductions.