Streaming-generated guards for backward-propagation prefetching look fishy, but appear to work

[FabioLuporini]

Complete discussion here: https://devitocodes.slack.com/archives/C7JMLMSG0/p1673960926948329

No MFE available right now

The issue discussion started here: https://github.com/devitocodes/devito/pull/2042/files#r1064672824

The latest suggestion from @mloubout was:

`p1-1 - (p1-1 + p0*factor)%factor`  this should always work. Basically always compute the modulo as the left part making sure it's positive by shifting it. This a bit brute force but a trivial shift multiple of factor

However, I tried it, and there were failures in PRO.