[IsKnownNonZero] Handle the case with non-constant phi nodes

Handle the case when all inputs of phi are proven to be non zero.

Constants are checked in beginning of this method before check for depth of recursion,
so it is a partial case of non-constant phi.

Recursion depth is already handled by the function.

Reviewers: aqjune, nikic, efriedma
Reviewed By: nikic
Subscribers: dantrushin, hiraditya, jdoerfert, llvm-commits
Differential Revision: https://reviews.llvm.org/D88276