Various cleanups to Layout (#6042)

* Use inst_block instead of pp_block where possible

* Remove unused is_block_gap method

* Remove ProgramOrder trait

It only has a single implementation

* Rename Layout::cmp to pp_cmp to distinguish it from Ord::cmp

* Make pp_block non-generic

* Use rpo_cmp_block instead of rpo_cmp in the verifier

* Remove ProgramPoint

* Rename ExpandedProgramPoint to ProgramPoint

* Remove From<ValueDef> for ProgramPoint impl

cranelift/codegen/src/legalizer/mod.rs

@@ -299,7 +299,10 @@ fn expand_cond_trap(
     //
     //   new_block_resume:
     //     ..
-    let old_block = func.layout.pp_block(inst);
+    let old_block = func
+        .layout
+        .inst_block(inst)
+        .expect("Instruction not in layout.");
     let new_block_trap = func.dfg.make_block();
     let new_block_resume = func.dfg.make_block();