Implement IntoIterator for PrimaryMap.

This makes iterating through a `PrimaryMap` more consistent with
iterating through a `Vec`.

lib/codegen/src/dce.rs

@@ -44,7 +44,7 @@ pub fn do_dce(func: &mut Function, domtree: &mut DominatorTree) {
     let mut live = Vec::with_capacity(func.dfg.num_values());
     live.resize(func.dfg.num_values(), false);
 
-    for &ebb in domtree.cfg_postorder().iter() {
+    for &ebb in domtree.cfg_postorder() {
         let mut pos = FuncCursor::new(func).at_bottom(ebb);
         while let Some(inst) = pos.prev_inst() {
             {

lib/codegen/src/regalloc/pressure.rs

@@ -88,7 +88,7 @@ impl Pressure {
         };
 
         // Get the layout of aliasing top-level register classes from the register banks.
-        for bank in reginfo.banks.iter() {
+        for bank in reginfo.banks {
             let first = bank.first_toprc;
             let num = bank.num_toprcs;
 

lib/codegen/src/write.rs

@@ -50,31 +50,31 @@ fn write_preamble(
         writeln!(w, "    {} = {}", ss, slot)?;
     }
 
-    for (gv, gv_data) in func.global_vars.iter() {
+    for (gv, gv_data) in &func.global_vars {
         any = true;
         writeln!(w, "    {} = {}", gv, gv_data)?;
     }
 
-    for (heap, heap_data) in func.heaps.iter() {
+    for (heap, heap_data) in &func.heaps {
         any = true;
         writeln!(w, "    {} = {}", heap, heap_data)?;
     }
 
     // Write out all signatures before functions since function declarations can refer to
     // signatures.
-    for (sig, sig_data) in func.dfg.signatures.iter() {
+    for (sig, sig_data) in &func.dfg.signatures {
         any = true;
         writeln!(w, "    {} = {}", sig, sig_data.display(regs))?;
     }
 
-    for (fnref, ext_func) in func.dfg.ext_funcs.iter() {
+    for (fnref, ext_func) in &func.dfg.ext_funcs {
         any = true;
         if ext_func.signature != SigRef::reserved_value() {
             writeln!(w, "    {} = {}", fnref, ext_func)?;
         }
     }
 
-    for (jt, jt_data) in func.jump_tables.iter() {
+    for (jt, jt_data) in &func.jump_tables {
         any = true;
         writeln!(w, "    {} = {}", jt, jt_data)?;
     }