[llvm] RegisterCoalescer: Enable terminal rule by default (PR #161621)

[Quentin Colombet via llvm-commits]

https://github.com/qcolombet commented:

To give an opportunity for targets to adopt the terminal rule on their own, I think it make sense to have a target hook for it, like what we do with `join-globalcopies` and `::enableJoinGlobalCopies`

I don't remember why I didn't flip the switch when I introduced this rule, so I want to make sure we are friendly with downstream users.

https://github.com/llvm/llvm-project/pull/161621