Use CursorBase utility functions to reduce repositioning clutter.

lib/cretonne/src/legalizer/boundary.rs

@@ -63,8 +63,7 @@ fn legalize_entry_arguments(func: &mut Function, entry: Ebb) {
     // 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_top(entry);
-    pos.next_inst();
+    pos.goto_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/split.rs

@@ -235,8 +235,7 @@ fn split_value(
                 //
                 // Note that it is safe to move `pos` here since `reuse` was set above, so we don't
                 // need to insert a split instruction before returning.
-                pos.goto_top(ebb);
-                pos.next_inst();
+                pos.goto_first_inst(ebb);
                 dfg.ins(pos).with_result(value).Binary(
                     concat,
                     split_type,