Handle a half-full func.locations map.

If func.locations has not been properly resized to have an entry for all
values, we should just full in the default location for the missing
values instead of crashing.

lib/cretonne/src/write.rs

@@ -186,7 +186,9 @@ fn write_instruction(w: &mut Write,
             if !func.locations.is_empty() {
                 let regs = isa.register_info();
                 for r in func.dfg.inst_results(inst) {
-                    write!(s, ",{}", func.locations[r].display(&regs))?
+                    write!(s,
+                           ",{}",
+                           func.locations.get(r).cloned().unwrap_or_default().display(&regs))?
                 }
             }
             write!(s, "]")?;