[InstCombine] Fold dependent IVs (#81151)

Fold `iv = phi(start, iv.next = iv2.next + start)` where `iv2 =
phi(iv2.start, iv2.next = iv2 + iv2.step)`
to  `iv = iv2 + start` removing one induction variable from the loop.

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

Fixes https://github.com/llvm/llvm-project/issues/77108.