Modifies the index instruction from `gep [0 x %Type]` to `gep %Type`
Fixes#133979.
This PR modifies the index instruction from `gep [0 x %Type]` to `gep %Type`, which is the same with pointer offset calculation.
This will help LLVM calculate various formats of GEP instructions. According to [[RFC] Replacing getelementptr with ptradd](https://discourse.llvm.org/t/rfc-replacing-getelementptr-with-ptradd/68699), we ultimately aim to canonicalize everything to `gep i8`. Based on the results from https://github.com/rust-lang/rust/pull/134117#issuecomment-2531717076, I think we still need to investigate some missing optimizations, so this PR is just a small step forward.
r? compiler
a611773d80