[llvm] [AMDGPU] Verify dominance when rewriting spills to registers (PR #167347)

[via llvm-commits]

ronlieb wrote:

> Can the test case be reduced further?
> 
> Can we have a unit test exercises this one pass alone (i.e., .mir)?
> 
> Generally speaking, I'm not a fan of test that only check "do not crash"

@qcolombet   Harhsa has spent days trying to reduce this test case to little avail, and we are at wits end. Could you help out here?

https://github.com/llvm/llvm-project/pull/167347