[llvm] [DRAFT][RegisterCoalescer] Enable non-trivial rematerialization (PR #160153)

[Philip Reames via llvm-commits]

preames wrote:

> Where `isTriviallyReMaterializable` would check for virtual registers. We'd then go through and replace existing uses of `isTriviallyReMaterializable` in the first group of users you identified (where they check for use availability) with `isReMaterializable`

I'll throw up a patch tomorrow which does something along these lines as an NFC such that we can then go through call site by call site and make adjustment as warranted.  

https://github.com/llvm/llvm-project/pull/160153