uniqFromMask Returns Duplicate Uniques with -jN, N > 1

uniqFromMask can return duplicate uniques when running stage 1 GHC with more than one thread. There is a primitive test here to verify this: tmobile/ghc@fb6c708f. This seems to have nothing to do with cross-compilation as originally suspected; cross compilers are always stage 1 compilers, cross-compilation targeting aarch64 uses LLVM, and opt simply complains in a way that reveals the issue.

Original description:

When cross-compiling packages (from x86_64 to aarch64, with GHC 8.8.x) I sometimes see errors of the form:

opt: /build/ghc119_0/ghc_103.ll:997:3: error: multiple definition of local value named 'ln5hca'
  %ln5hca = inttoptr i64 %ln5hca to i64*
  ^

<no location info>: error:
    `opt' failed in phase `LLVM Optimiser'. (Exit code: 1)

It seems that GHC sometimes uses stale names when generating IR, but I have no idea what this might have to do with cross-compilation. I have never seen this with a native build on Intel or aarch64. Since names are generated randomly (as far as I remember), the error may often be bypassed by simply trying again. Naturally, packages with large modules are more likely to encounter this issue.

Previously, it would've been a pain to setup a cross compiler to reproduce this, but thanks to IOHK's excellent work at https://github.com/input-output-hk/haskell.nix , this is now quite easy.