Find an appropriate way to deal with 0-bit types being converted to LLVM

[cdleary]

These may only be present in unopt builds, but we still want to be able to JIT compile unopt IR. Simplest example I believe is:

concat.1: bits[0] = concat()

For now I'll just say max(bit_count, 1) to avoid making LLVM assertion fail in the type conversion.

[cdleary]

This could be a reasonable first bug for somebody trying to learn the ropes of the JIT infrastructure and make a "somewhat small in design scope but still cross-cutting" modification that would teach the structure of our conversions from XLS IR to LLVM IR.