[llvm] [ISel] Replace expensive move from wzr by two moves via floating point immediate (PR #146538)

[David Green via llvm-commits]

https://github.com/davemgreen commented:

> Sure, do you maybe have an example I can work off, not sure how to go about this.

I think that DisableFastIncVL/HasFastIncVL shows how to add a subtarget feature and make patterns dependant on it. The old patterns can be made dependant on it, I believe, and it can be added to the processors that would not benefit. (FastIncVL is perhaps a bit odd as it doesn't appear to be used anywhere).

https://github.com/llvm/llvm-project/pull/146538