[llvm] [AMDGPU] Use absolute relocations when compiling for AMDPAL and Mesa3D (PR #67791)

[Nicolai Hähnle via llvm-commits]

nhaehnle wrote:

My understanding is:

* We should be accepting to already existing registers having a register class for some reason
* But MergeValues works with any register class, so doesn't necessarily need a fresh register
* We shouldn't need Casts, only COPYs

https://github.com/llvm/llvm-project/pull/67791