[ValueTracking] Fix analyses to update CxtI to be phi's incoming edges' terminators
It was mentioned that D88276 that when a phi node is visited, terminators at their incoming edges should be used for CtxI.
This is a patch that makes two functions (ComputeNumSignBitsImpl, isGuaranteedNotToBeUndefOrPoison) to do so.
Reviewed By: nikic
Differential Revision: https://reviews.llvm.org/D88360