[llvm] [RegAlloc] Account for use availability when applying rematerializable weight discount (PR #159180)

[Philip Reames via llvm-commits]

https://github.com/preames commented:

Remember to update the comment on the call to isRematerializable

We could consider a deeper rework of this.  The 0.5 multiply is somewhat arbitrary, and we could consider e.g. scaling that decrease by the proportion of defs which are materialized.  There's a bunch of option for playing with this if you want.  

https://github.com/llvm/llvm-project/pull/159180