[X86] Add test case to show missed opportunity to turn (add (zext (vXi1 X)),...

[X86] Add test case to show missed opportunity to turn (add (zext (vXi1 X)), Y) -> (sub Y, (sext (vXi1 X))) with avx512.

With avx512, the vXi1 type is legal. And we can more easily sign
extend them to vector registers. zext requires a sign extend and
a shift.

If we can easily turn the zext into a sext we should.

llvm-svn: 373131