[aarch64] Unguard GEPOpt from O3

This chunk of code currently runs only if the optimization mode is O3
AND the EnableGEPOpt flag is set. Given that this is the only use case
for the EnableGEPOpt flag, the guarding against O3 is kinda pointless.
IF the user wants to enable it then the flag should be sufficient.

Reviewers: TNorthover, aeubanks

Reviewed By: aeubanks

Pull Request: https://github.com/llvm/llvm-project/pull/86588