Provide a way to statically elide unreachable errors

Per slack:

Charlie Kawczynski
> Can unreachable errors increase register usage?

Tim Besard
> yes. registers are statically allocated, so the compiler needs to take all possible paths into account. so unless it's been optimized away (check the generated LLVM IR), they may affect register usage.

So, this may be impacting our gpu register usage.

The issue names no files or tests. Start by reproducing the affected GPU code and inspecting its generated LLVM IR, as suggested in the discussion, to determine whether unreachable errors remain and affect register allocation. Done should include a defined way to elide those errors and evidence of the resulting register usage.

Written by the indexing model from the issue text.