[llvm] RegAlloc: Do not fatal error if there are no registers in the alloc order (PR #119640)

[Thurston Dang via llvm-commits]

thurstond wrote:

Hi! Two of the tests are still failing on the buildbot (https://lab.llvm.org/buildbot/#/builders/52/builds/4539), even after your fix "RegAllocBase: Avoid using temporary DiagnosticInfo" (https://github.com/llvm/llvm-project/pull/120046):
```
  LLVM :: CodeGen/AMDGPU/ran-out-of-registers-error-all-regs-reserved.ll
  LLVM :: CodeGen/AMDGPU/ran-out-of-registers-errors.ll
```

https://github.com/llvm/llvm-project/pull/119640