amdgcn: Resolve insn conditions at compile time

amdgcn: Resolve insn conditions at compile time

GET_MODE_NUNITS isn't a compile time constant, so we end up with many
impossible insns in the machine description.  Adding MODE_VF allows the insns
to be eliminated completely.

gcc/ChangeLog:

	* config/gcn/gcn-valu.md
	(<cvt_name><VCVT_MODE:mode><VCVT_FMODE:mode>2<exec>): Use MODE_VF.
	(<cvt_name><VCVT_FMODE:mode><VCVT_IMODE:mode>2<exec>): Likewise.
	* config/gcn/gcn.h (MODE_VF): New macro.