[llvm] [KnownBits] Fix add() SelfAdd assertion for bitwidths >= 512 (PR #202769)

[Paweł Bylica via llvm-commits]

chfast wrote:

Good point, thanks. Moved the fix into `getMaxShiftAmount` as suggested:

```cpp
return MaxValue.extractBitsAsZExtValue(
    std::min(Log2_32(BitWidth), MaxValue.getBitWidth()), 0);
```

When the shift amount is narrower than `Log2_32(BitWidth)` bits, its value is necessarily `< BitWidth`, so clamping the extraction width keeps it a valid upper bound. This protects all three shift callers, and `KnownBits::add`'s 8-bit self-add amount is reverted to its original form.

https://github.com/llvm/llvm-project/pull/202769