Rollup merge of #107022 - scottmcm:ordering-option-eq, r=m-ou-se

Rollup merge of #107022 - scottmcm:ordering-option-eq, r=m-ou-se

Implement `SpecOptionPartialEq` for `cmp::Ordering`

Noticed as I continue to explore options for having code using `partial_cmp` optimize better.

Before:
```llvm
; Function Attrs: mustprogress nofree nosync nounwind willreturn uwtable
define noundef zeroext i1 `@ordering_eq(i8` noundef %0, i8 noundef %1) unnamed_addr #0 {
start:
  %2 = icmp eq i8 %0, 2
  br i1 %2, label %bb1.i, label %bb3.i

bb1.i:                                            ; preds = %start
  %3 = icmp eq i8 %1, 2
  br label %"_ZN55_$LT$T$u20$as$u20$core..option..SpecOptionPartialEq$GT$2eq17hb7e7beacecde585fE.exit"

bb3.i:                                            ; preds = %start
  %.not.i = icmp ne i8 %1, 2
  %4 = icmp eq i8 %0, %1
  %spec.select.i = and i1 %.not.i, %4
  br label %"_ZN55_$LT$T$u20$as$u20$core..option..SpecOptionPartialEq$GT$2eq17hb7e7beacecde585fE.exit"

"_ZN55_$LT$T$u20$as$u20$core..option..SpecOptionPartialEq$GT$2eq17hb7e7beacecde585fE.exit": ; preds = %bb1.i, %bb3.i
  %.0.i = phi i1 [ %3, %bb1.i ], [ %spec.select.i, %bb3.i ]
  ret i1 %.0.i
}
```

After:
```llvm
; Function Attrs: mustprogress nofree norecurse nosync nounwind readnone willreturn uwtable
define noundef zeroext i1 `@ordering_eq(i8` noundef %0, i8 noundef %1) unnamed_addr #1 {
start:
  %2 = icmp eq i8 %0, %1
  ret i1 %2
}
```

(Which <https://alive2.llvm.org/ce/z/-rop5r> says LLVM *could* just do itself, but there's probably an issue already open for that problem from when this was originally looked at for `Option<NonZeroU8>` and friends.)