[llvm-bugs] [Bug 38146] New: `add nsw` + `sadd.with.overflow` with constants should fold

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=38146

            Bug ID: 38146
           Summary: `add nsw` + `sadd.with.overflow` with constants should
                    fold
           Product: new-bugs
           Version: unspecified
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: me22.ca at gmail.com
                CC: llvm-bugs at lists.llvm.org

```
  %3 = add nsw i32 %1, 7
  %4 = tail call { i32, i1 } @llvm.sadd.with.overflow.i32(i32 %3, i32 13)
```

should be folded to

```
  %4 = tail call { i32, i1 } @llvm.sadd.with.overflow.i32(i32 %1, i32 20)
```

Repro in clang: https://godbolt.org/g/DM5Gq9

```
#include <optional>

std::optional<int> test_nsw_sadd_fold(int num) {
    num += 7;
    int r;
    if (__builtin_sadd_overflow(num, 13, &r)) {
        return {};
    }
    return r;
}
```

Cases for `.add_nsw(C1).sadd_overflow(C2)`:

- Same sign can be folded to a single overflow call
- Opposite signs with |C2|>|C1| can be folded to a single overflow call
- Opposite signs with |C2|<=|C1| can be folded to a single nsw call

(Similarly, just simpler, for `.add_nuw(C1).uadd_overflow(C2)`.)

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