Move return_at_end out of Settings and into the wasm FuncEnvironment. (#547)

* Move `return_at_end` out of Settings and into the wasm FuncEnvironment.

The `return_at_end` flag supports users that want to append a custom
epilogue to Cranelift-produced functions. It arranges for functions to
always return via a single return statement at the end, and users are
expected to remove this return to append their code.

This patch makes two changes:
 - First, introduce a `fallthrough_return` instruction and use that
   instead of adding a `return` at the end. That's simpler than having
   users remove the `return` themselves.

 - Second, move this setting out of the Settings and into the wasm
   FuncEnvironment. This flag isn't something the code generator uses,
   it's something that the wasm translator uses. The code generator
   needs to preserve the property, however we can give the
   `fallthrough_return` instruction properties to ensure this as needed,
   such as marking it non-cloneable.

lib/wasm/tests/wasm_testsuite.rs

@@ -6,9 +6,9 @@ extern crate wabt;
 
 use cranelift_codegen::isa;
 use cranelift_codegen::print_errors::pretty_verifier_error;
-use cranelift_codegen::settings::{self, Configurable, Flags};
+use cranelift_codegen::settings::{self, Flags};
 use cranelift_codegen::verifier;
-use cranelift_wasm::{translate_module, DummyEnvironment};
+use cranelift_wasm::{translate_module, DummyEnvironment, ReturnMode};
 use std::fs;
 use std::fs::File;
 use std::io;
@@ -35,16 +35,18 @@ fn testsuite() {
     let flags = Flags::new(settings::builder());
     for path in paths {
         let path = path.path();
-        handle_module(&path, &flags);
+        handle_module(&path, &flags, ReturnMode::NormalReturns);
     }
 }
 
 #[test]
-fn return_at_end() {
-    let mut flag_builder = settings::builder();
-    flag_builder.enable("return_at_end").unwrap();
-    let flags = Flags::new(flag_builder);
-    handle_module(Path::new("../../wasmtests/return_at_end.wat"), &flags);
+fn use_fallthrough_return() {
+    let flags = Flags::new(settings::builder());
+    handle_module(
+        Path::new("../../wasmtests/use_fallthrough_return.wat"),
+        &flags,
+        ReturnMode::FallthroughReturn,
+    );
 }
 
 fn read_file(path: &Path) -> io::Result<Vec<u8>> {
@@ -54,7 +56,7 @@ fn read_file(path: &Path) -> io::Result<Vec<u8>> {
     Ok(buf)
 }
 
-fn handle_module(path: &Path, flags: &Flags) {
+fn handle_module(path: &Path, flags: &Flags, return_mode: ReturnMode) {
     let data = match path.extension() {
         None => {
             panic!("the file extension is not wasm or wat");
@@ -73,7 +75,9 @@ fn handle_module(path: &Path, flags: &Flags) {
             None | Some(&_) => panic!("the file extension for {:?} is not wasm or wat", path),
         },
     };
-    let mut dummy_environ = DummyEnvironment::with_triple_flags(triple!("riscv64"), flags.clone());
+    let mut dummy_environ =
+        DummyEnvironment::with_triple_flags(triple!("riscv64"), flags.clone(), return_mode);
+
     translate_module(&data, &mut dummy_environ).unwrap();
 
     let isa = isa::lookup(dummy_environ.info.triple)