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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> 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 specifically think we should not have a control for this, and have a policy against introducing this style of target hook. I think these are lazy shortcuts to avoid touching targets whoever adds the feature doesn't care about, and the project is worse off for it.  The reality of target maintenance is this will never be implemented by any target. If we wait several years, a few might discover it and turn it off 

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