goldsteinn 1e072ae289 [CGP] [CodeGenPrepare] Folding urem with loop invariant value plus offset (#104724) ...

This extends the existing fold:

```
for(i = Start; i < End; ++i)
   Rem = (i nuw+- IncrLoopInvariant) u% RemAmtLoopInvariant;
```
 ->
```
Rem = (Start nuw+- IncrLoopInvariant) % RemAmtLoopInvariant;
for(i = Start; i < End; ++i, ++rem)
   Rem = rem == RemAmtLoopInvariant ? 0 : Rem;
```

To work with a non-zero `IncrLoopInvariant`.

This is a common usage in cases such as:

```
for(i = 0; i < N; ++i)
    if ((i + 1) % X) == 0)
        do_something_occasionally_but_not_first_iter();
```

Alive2 w/ i4/unrolled 6x (needs to be ran locally due to timeout):
https://alive2.llvm.org/ce/z/6tgyN3

Exhaust proof over all uint8_t combinations in C++:
https://godbolt.org/z/WYa561388

2024-10-31 09:14:33 -05:00

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