[InstCombine] Canonicalize `gep T, (gep i8, base, C1), (Index + C2)` into `gep...

[InstCombine] Canonicalize `gep T, (gep i8, base, C1), (Index + C2)` into `gep T, (gep i8, base, C1 + C2 * sizeof(T)), Index` (#76177)

This patch tries to canonicalize `gep T, (gep i8, base, C1), (Index +
C2)` into `gep T, (gep i8, base, C1 + C2 * sizeof(T)), Index`.

Alive2: https://alive2.llvm.org/ce/z/dxShKF
Fixes regressions found in
https://github.com/llvm/llvm-project/pull/68882.