[ValueTracking] Bail out on non-immediate constant expressions (#168084) In https://github.com/llvm/llvm-project/pull/165748 constant expressions were allowed in `collectPossibleValues` because we are still using insertelement + shufflevector idioms to represent a scalable vector splat. However, it also accepts some unresolved constants like ptrtoint of globals or pointer difference between two globals. Absolutely we can ask the user to check this case with the constant folding API. However, since we don't observe the real-world usefulness of handling constant expressions, I decide to be more conservative and only handle immediate constants in the helper function. With this patch, we don't need to touch the SimplifyCFG part, as the values can only be either ConstantInt or undef/poison values (NB: switch on undef condition is UB). Fix the miscompilation reported by https://github.com/llvm/llvm-project/pull/165748#issuecomment-3532245218
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