[llvm] [AutoUpgrade] Prevent deletion of call if uses still exist (PR #177606)

[Nikita Popov via llvm-commits]

nikic wrote:

Ultimately the problem here is a mismatch with shouldUpgradeX86Intrinsic. I don't think making them match exactly is a good use of anyone's time, so I'd prefer liberal use of reportFatalUsageError() for intrinsics that look like one's that have existed in the past, but did not actually.

So that means replacing all of those llvm_unreachable with reportFatalUsageError() and also adding an else {} with it (I assume that's where we end up for this specific case).

https://github.com/llvm/llvm-project/pull/177606