Optimize SbTag::eq

The code before generated really bad code with a branch.
This nudges LLVM towards being smarter and simply comparing
the integers.

src/stacked_borrows.rs

@@ -27,12 +27,27 @@
 pub type AllocExtra = Stacks;
 
 /// Tracking pointer provenance
-#[derive(Copy, Clone, Hash, PartialEq, Eq)]
+#[derive(Copy, Clone, Hash, Eq)]
 pub enum SbTag {
     Tagged(PtrId),
     Untagged,
 }
 
+impl SbTag {
+    fn as_u64(self) -> u64 {
+        match self {
+            SbTag::Tagged(id) => id.get(),
+            SbTag::Untagged => 0,
+        }
+    }
+}
+
+impl PartialEq for SbTag {
+    fn eq(&self, other: &Self) -> bool {
+        self.as_u64() == other.as_u64()
+    }
+}
+
 impl fmt::Debug for SbTag {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {