AVX-512: Generate KTEST instead of TEST fir i1 vectors

KTEST instruction may be used instead of TEST in this case:

%int_sel3 = bitcast <8 x i1> %sel3 to i8
%res = icmp eq i8 %int_sel3, zeroinitializer
br i1 %res, label %L2, label %L1

Differential Revision: http://reviews.llvm.org/D18444

llvm-svn: 264298