[ValueTracking] Fix crash in isGuaranteedNotToBeUndefOrPoison when V is in an unreachable block

Summary:
This fixes PR45885 by fixing isGuaranteedNotToBeUndefOrPoison so it does not look into dominating
branch conditions of V when V is an instruction in an unreachable block.

Reviewers: spatel, nikic, lebedev.ri

Reviewed By: nikic

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79790