Silence "unreachable loop" warning for GPU builds (#3462)

This silences warnings like `Src/Base/AMReX_PODVector.H(126): warning
#128-D: loop is not reachable`

The proposed changes:
- [ ] fix a bug or incorrect behavior in AMReX
- [ ] add new capabilities to AMReX
- [ ] changes answers in the test suite to more than roundoff level
- [ ] are likely to significantly affect the results of downstream AMReX
users
- [ ] include documentation in the code and/or rst files, if appropriate

Src/Base/AMReX_PODVector.H

@@ -122,8 +122,12 @@ namespace amrex
  return;
  }
  }
+#else
+ static_assert(RunOnGpu<Allocator<T>>::value == false);
 #endif
- for (Size i = 0; i < count; ++i) { dst[i] = src[i]; }
+ if constexpr (! RunOnGpu<Allocator<T>>::value) {
+ for (Size i = 0; i < count; ++i) { dst[i] = src[i]; }
+ }
  }
 
  template <typename Allocator>