[llvm] [MemCpyOpt] Fix the invalid code modification for GEP (PR #68479)

[via llvm-commits]

lcvon007 wrote:

> Instead of rolling back, we should not do the change until we know the transform will succeed. The simplest way to do that would be to move this check to the end. (Setting a variable to perform the move later would also work.)

please update commit message too.

https://github.com/llvm/llvm-project/pull/68479