[InstCombine] Canonicalize `gep T* X, V / sizeof(T)` to `gep i8* X, V` (#76458)

This patch canonicalize `gep T* X, V / sizeof(T)` to `gep i8* X, V`.
Alive2: https://alive2.llvm.org/ce/z/7XGjiB

As this pattern has been handled by the backends, the motivation of this
patch is to reduce the ref count of sdiv, which will enable more
optimizations.