amdgcn: bug fix ldexp insn

The vop3 instructions don't support B constraint immediates.
Also, take the use the SV_FP iterator to delete a redundant pattern.

gcc/ChangeLog:

	* config/gcn/gcn-valu.md (vnsi, VnSI): Add scalar modes.
	(ldexp<mode>3): Delete.
	(ldexp<mode>3<exec>): Change "B" to "A".