[KnowFPClass] Refactor fmul & fdiv (NFC) (#191651)
- Remove `0 * Inf -> NaN` redundant check and a nan setting.
The reason:
```rust
1. Inf * Y -> {Inf, NaN}
2. 0 * Y -> {Zero, NaN}
3. 0 * Inf -> NaN
```
But after `1.` and `2.` we already have `{Inf, NaN} ∩ {Zero, NaN} ->
NaN`
So `3.` is redundant and [can be
removed](https://github.com/llvm/llvm-project/pull/191651/changes#diff-eacbd8c8620db92a00453be14b3b433c618946ad7b57c10b039437641e9777c4L389):
```diff
// +/-0 * +/-inf = nan
- if ((KnownLHS.isKnownAlways(fcZero | fcNan) &&
- KnownRHS.isKnownAlways(fcInf | fcNan)) ||
- (KnownLHS.isKnownAlways(fcInf | fcNan) &&
- KnownRHS.isKnownAlways(fcZero | fcNan)))
- Known.knownNot(~fcNan);
```
- Group repeated twice sign inferring into `propagateXorSign` helper
- Add more conventional comments
