GHCi unboxed tuples register saving incompatible with LLVM

Unfortunately this wasn't picked up by CI. The stg_ctoi_t and stg_ret_t closures save and restore a tuple from registers, respectively. They use a tuple_info word that describes which registers are live. Overlapping registers are never live at the same time, but the LLVM code generator doesn't know this, resulting in errors like this:

ghc-stage1: sorry! (unimplemented feature or known bug)
  (GHC version 9.1.20210325:
	LLVM code generator
  Found two different Cmm registers (F1,D1) both alive AND mapped to the same real register: %r17. This isn't currently supported by the LLVM backend.

We probably need to change the Cmm register saving strategy a little bit.

Edited Mar 25, 2021 by Luite Stegeman

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