[PowerPC] Make specialized AltiVec patterns isCodeGenOnly

A couple of AltiVec patterns are just specialized forms of the
generic instruction pattern, and should therefore be marked
isCodeGenOnly to avoid confusing the asm parser:
VCFSX_0, VCTUXS_0, VCFUX_0, VCTSXS_0, and V_SETALLONES.

Noticed by inspection of the generated PPCGenAsmMatcher.inc.