[llvm] [CodeGen] Gate rematerializer unit tests on AMDGPU target being enabled (PR #202966)

[Lucas Ramirez via llvm-commits]

lucas-rami wrote:

Yes this currently happens in the `SetUp` instead of `SetUpTestCase`

```cpp
const Target *T = TargetRegistry::lookupTarget("", TT, Error);
if (!T)
  GTEST_SKIP();
```

Adding an `#else` section with `GTEST_SKIP();` after the `#if LLVM_HAS_AMDGPU_TARGET` section achieves the same effect and avoid querying the same missing target many times, so I just added one.

https://github.com/llvm/llvm-project/pull/202966