[llvm] [RegAlloc] Remove default restriction on non-trivial rematerialization (PR #159211)

[Luke Lau via llvm-commits]

lukel97 wrote:

Thanks for reporting this @alexfh, a reproducer would definitely be much appreciated. In the meantime you can probably work around it in your target's `isRematerializableImpl` by appending something like

```
for (auto &MO : MI.uses())
  if (MO.isReg() && MO.getReg() && MO.getReg().isVirtual())
    return false;
```

Which should restore the previous behaviour.

@preames Should we revert this now or wait until we get a reproducer first? I will be travelling tomorrow but feel free to go ahead and revert on my behalf if needed. 

https://github.com/llvm/llvm-project/pull/159211