[DAGCombiner] Fold add (mul x, C), x to mul x, C+1

While this is normally non-canonical IR, this pattern can appear
during SDAG lowering if the add is actually a getelementptr, as
illustrated in `@test_ptr`. This pattern comes up when doing
provenance-aware high-bit pointer tagging.

Proof: https://alive2.llvm.org/ce/z/DLoEcs

Fixes https://github.com/llvm/llvm-project/issues/62093.

Differential Revision: https://reviews.llvm.org/D148341