[X86][AVX512] Fix PR31515 - Do not flip vselect condition if it's not a vXi1 mask

[X86][AVX512] Fix PR31515 - Do not flip vselect condition if it's not a vXi1 mask

r289653 added a case where `vselect <cond> <vector1> <all-zeros>`
is transformed to:
`vselect xor(cond, DAG.getConstant(1, DL, CondVT) <all-zeros> <vector1>`
This was not aimed to catch cases where Cond is not a vXi1
mask but it does. Moreover, when Cond type is VxiN (N > 1)
then xor(cond, DAG.getConstant(1, DL, CondVT) != NOT(cond).
This patch changes the above to xor with allones, and avoids
entering the case for non-mask Conds.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@291745 91177308-0d34-0410-b5e6-96231b3b80d8