[InstCombine] Don't look at ConstantData users
When looking at PHI operand for combining, only look at instructions and
arguments. The loop later iteraters over Arg's users, which is not
useful if Arg is a constant -- it's users are not meaningful and might
be in different functions, which causes problems for the dominates()
query.
Pull Request: https://github.com/llvm/llvm-project/pull/103302
