[AutoUpgrade] Fix length check for intrinsic upgrade

The shortest intrinsics that can be upgraded via remangling have
8 characters (like "llvm.abs"). Make sure these go through the
upgrade code.

I think that currently this change is not observable from in-tree
callers of UpgradeIntrinsicFunction(), because callers do
redundant remangling checks. However, this issue shows up in
existing tests if those checks are removed (which I will do in
followup changes).