[InstCombine] Fold (A & 2^C1) + A => A & (2^C1 - 1) iff bit C1 in A is a sign bit (PR21929)

Alive2: https://alive2.llvm.org/ce/z/Ygq26C

This is the final missing fold to handle the modulo2 simplification: https://github.com/llvm/llvm-project/issues/22303

Fixes #22303

Differential Revision: https://reviews.llvm.org/D123374