[InstCombine] Simplify nonnull pointers (#128111)
This patch is the follow-up of
https://github.com/llvm/llvm-project/pull/127979. It introduces a helper
`simplifyNonNullOperand` to avoid duplicate logic. It also addresses the
one-use issue in `visitLoadInst`, as discussed in
https://github.com/llvm/llvm-project/pull/127979#issuecomment-2671013972.
The `nonnull` attribute is also supported. Proof:
https://alive2.llvm.org/ce/z/MCKgT9
