Implement SpecOptionPartialEq for cmp::Ordering

[scottmcm]

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

Before:

; 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:

; 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.)

[rustbot]

r? @m-ou-se

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[m-ou-se]

It seems worrying that rustc/llvm didn't optimize this.

but there's probably an issue already open for that problem

This PR seems fine for now (r=me), but can you please open an issue (if there isn't one already) and add a comment to SpecOptionPartialEq linking to that issue? It'd be great to be able to get rid of SpecOptionPartialEq again at some point.

[scottmcm]

Issue found; Comment added; CI passed.

@bors r=m-ou-se

[bors]

📌 Commit 3e9d1e4 has been approved by m-ou-se

It is now in the queue for this repository.