This is just one part of the MCP, but it's the one that IMHO removes the most noise from the standard library code.
Seems net simpler this way, since MIR already supported heterogeneous shifts anyway, and thus it's not more work for backends than before.
To reproduce the changes in this commit locally:
- Run `./x test tidy` and remove all the output files not associated
with a test file anymore, as reported by tidy.
- Run `./x test tests/mir-opt --bless` to generate the new outputs.
