intrusive_list: Make Remove and IsEmpty stdlib compatible

Makes the name match the standard library equivalents.

C++17 introduces non-member empty() which allows for nicer handling
in generic contexts. May as well make the data structure compatible with
it.

src/ir_opt/dead_code_elimination_pass.cpp

@@ -61,7 +61,7 @@ void DeadCodeElimination(IR::Block& block) {
     // We iterate over the instructions in reverse order.
     // This is because removing an instruction reduces the number of uses for earlier instructions.
 
-    if (block.instructions.IsEmpty()) {
+    if (block.instructions.empty()) {
         return;
     }