[X86] Separate the call to LowerVectorAllZeroTest from EmitTest. NFCI

Every instruction that has the word TEST in its name seems to have been buried into EmitTest. But that code is largely concerned with trying to reuse the flags from instructions that update flags in a pretty normal way.

PTEST/TESTP/KTEST do not update flags in a normal way. They only update Z and C and the C flag update is non-standard. Rather than try to bend EmitTest's already complex logic to accomodate this, just move the call up to LowerSETCC and replicate the few pre-checks that are needed.

While there add a FIXME for using the C flag for checking for all 1s which we definitely couldn't do from EmitTEST.

llvm-svn: 324029