[ValueTracking] Let getGuaranteedNonPoisonOp find multiple non-poison operands

This patch helps getGuaranteedNonPoisonOp find multiple non-poison operands.

Instead of special-casing llvm.assume, I think it is also a viable option to
add noundef to Intrinsics.td. If it makes sense, I'll make a patch for that.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D86477