[llvm] [CodeGenSchedule] Don't allow invalid ReadAdvances to be formed (PR #82685)

[Evandro Menezes via llvm-commits]

ebahapo wrote:

@francisvm, thank you for this change. 

As a matter of fact, all the proprietary Exynos processors through M5 are obsolete and so is the support for them in LLVM. However, I’m not associated with Samsung anymore. 

https://github.com/llvm/llvm-project/pull/82685