[ValueTracking] Let isGuaranteedNotToBeUndefOrPoison look into branch conditions of dominating blocks' terminators
br i1 c, BB1, BB2: BB1: use1(c) BB2: use2(c)
In BB1 and BB2, c is never undef or poison because otherwise the branch would have triggered UB.
Checked with Alive2
Reviewers: xbolva00, spatel, lebedev.ri, reames, jdoerfert, nlopes, sanjoy
Reviewed By: reames
Subscribers: jdoerfert, hiraditya, llvm-commits
Differential Revision: https://reviews.llvm.org/D75401