amdgcn: Fix addsub bug

The vec_fmsubadd instuction actually had add twice, by mistake.

Also improve code-gen for all the complex patterns by using properly
undefined values.  Mostly this just prevents the compiler reserving space
in the stack frame.

gcc/ChangeLog:

	* config/gcn/gcn-valu.md (cmul<conj_op><mode>3): Use gcn_gen_undef.
	(cml<addsub_as><mode>4): Likewise.
	(vec_addsub<mode>3): Likewise.
	(cadd<rot><mode>3): Likewise.
	(vec_fmaddsub<mode>4): Likewise.
	(vec_fmsubadd<mode>4): Likewise, and use sub for the odd lanes.

(cherry picked from commit b17c57b0)