Use more helper functions on CursorBase to reduce cursor repositioning.

lib/cretonne/src/legalizer/boundary.rs

@@ -62,8 +62,7 @@ fn legalize_entry_arguments(func: &mut Function, entry: Ebb) {
     // Insert position for argument conversion code.
     // We want to insert instructions before the first instruction in the entry block.
     // If the entry block is empty, append instructions to it instead.
-    let mut pos = Cursor::new(&mut func.layout);
-    pos.goto_first_inst(entry);
+    let mut pos = Cursor::new(&mut func.layout).at_first_inst(entry);
 
     // Keep track of the argument types in the ABI-legalized signature.
     let abi_types = &func.signature.argument_types;

lib/cretonne/src/legalizer/mod.rs

@@ -137,8 +137,7 @@ fn expand_cond_trap(inst: ir::Inst, func: &mut ir::Function, cfg: &mut ControlFl
         func.dfg.replace(inst).brz(arg, new_ebb, &[]);
     }
 
-    let mut pos = FuncCursor::new(func).at_inst(inst);
-    pos.next_inst();
+    let mut pos = FuncCursor::new(func).after_inst(inst);
     pos.ins().trap();
     pos.insert_ebb(new_ebb);