Get rid of most spills

hercules_opt/src/gcm.rs

@@ -421,6 +421,9 @@ fn basic_blocks(
                 // If the next node further up the dominator tree is in a shallower
                 // loop nest or if we can get out of a reduce loop when we don't
                 // need to be in one, place this data node in a higher-up location.
+                // Only do this is the node isn't a constant or undef.
+                let is_constant_or_undef =
+                    function.nodes[id.idx()].is_constant() || function.nodes[id.idx()].is_undef();
                 let old_nest = loops
                     .header_of(location)
                     .map(|header| loops.nesting(header).unwrap());
@@ -441,7 +444,7 @@ fn basic_blocks(
                 // loop use the reduce node forming the loop, so the dominator chain
                 // will consist of one block, and this loop won't ever iterate.
                 let currently_at_join = function.nodes[location.idx()].is_join();
-                if shallower_nest || currently_at_join {
+                if !is_constant_or_undef && (shallower_nest || currently_at_join) {
                     location = control_node;
                 }
             }