Make sequence numbers local to instructions (#6043)

* Only allow pp_cmp within a single block

Block order shouldn't matter for codegen and restricting pp_cmp to a
single block will allow making instruction sequence numbers local to a
block.

* Make sequence numbers local to instructions

This allows renumbering to be localized to a single block where previously it
could affect the entire function. Also saves 32bit of overhead per block.

cranelift/codegen/src/dominator_tree.rs

@@ -106,7 +106,7 @@ impl DominatorTree {
         let a = a.into();
         let b = b.into();
         self.rpo_cmp_block(layout.pp_block(a), layout.pp_block(b))
-            .then(layout.pp_cmp(a, b))
+            .then_with(|| layout.pp_cmp(a, b))
     }
 
     /// Returns `true` if `a` dominates `b`.
@@ -578,7 +578,7 @@ impl DominatorTreePreorder {
         let a = a.into();
         let b = b.into();
         self.pre_cmp_block(layout.pp_block(a), layout.pp_block(b))
-            .then(layout.pp_cmp(a, b))
+            .then_with(|| layout.pp_cmp(a, b))
     }
 }