[LoopInterchange] Drop nuw/nsw flags from reduction ops when interchanging (#148612)
Before this patch, when a reduction exists in the loop, the legality
check of LoopInterchange only verified if there exists a
non-reassociative floating-point instruction in the reduction
calculation. However, it is insufficient, because reordering integer
reductions can also lead to incorrect transformations. Consider the
following example:
```c
int A[2][2] = {
{ INT_MAX, INT_MAX },
{ INT_MIN, INT_MIN },
};
int sum = 0;
for (int i = 0; i < 2; i++)
for (int j = 0; j < 2; j++)
sum += A[j][i];
```
To make this exchange legal, we must drop nuw/nsw flags from the
instructions involved in the reduction operations.
This patch extends the legality check to correctly handle such cases. In
particular, for integer addition and multiplication, it verifies that
the nsw and nuw flags are set on involved instructions, and drop them
when the transformation actually performed. This patch also introduces
explicit checks for the kind of reduction and permits only those that
are known to be safe for interchange. Consequently, some "unknown"
reductions (at the moment, `FindFirst*` and `FindLast*`) are rejected.
Fix #148228
llvm-project
b3c293c5 - [LoopInterchange] Drop nuw/nsw flags from reduction ops when interchanging (#148612)
