Always require a Flags reference for verifying functions.

Add a settings::FlagsOrIsa struct which represents a flags reference and
optionally the ISA it belongs to. Use this for passing flags/isa
information to the verifier.

The verify_function() and verify_context() functions are now generic so
they accept either a &Flags or a &TargetISa argument.

Fix the return_at_end verifier tests which no longer require an ISA
specified. The signle "set return_at_end" flag setting now makes it to
the verifier even when no ISA is present to carry it.

lib/frontend/src/frontend.rs

@@ -631,6 +631,7 @@ mod tests {
     use cretonne::ir::types::*;
     use frontend::{ILBuilder, FunctionBuilder};
     use cretonne::verifier::verify_function;
+    use cretonne::settings;
 
     use std::u32;
 
@@ -727,7 +728,8 @@ mod tests {
             builder.seal_block(block1);
         }
 
-        let res = verify_function(&func, None);
+        let flags = settings::Flags::new(&settings::builder());
+        let res = verify_function(&func, &flags);
         // println!("{}", func.display(None));
         match res {
             Ok(_) => {}

lib/frontend/src/lib.rs

@@ -38,6 +38,7 @@
 //! use cretonne::entity::EntityRef;
 //! use cretonne::ir::{FunctionName, CallConv, Function, Signature, ArgumentType, InstBuilder};
 //! use cretonne::ir::types::*;
+//! use cretonne::settings;
 //! use cton_frontend::{ILBuilder, FunctionBuilder};
 //! use cretonne::verifier::verify_function;
 //! use std::u32;
@@ -133,7 +134,8 @@
 //!         builder.seal_block(block1);
 //!     }
 //!
-//!     let res = verify_function(&func, None);
+//!     let flags = settings::Flags::new(&settings::builder());
+//!     let res = verify_function(&func, &flags);
 //!     println!("{}", func.display(None));
 //!     match res {
 //!         Ok(_) => {}

lib/frontend/src/ssa.rs

@@ -589,6 +589,7 @@ mod tests {
     use cretonne::ir::types::*;
     use cretonne::verify_function;
     use cretonne::ir::instructions::BranchInfo;
+    use cretonne::settings;
     use ssa::SSABuilder;
     use std::u32;
 
@@ -1194,7 +1195,8 @@ mod tests {
             cur.goto_bottom(ebb1);
             func.dfg.ins(cur).return_(&[])
         };
-        match verify_function(&func, None) {
+        let flags = settings::Flags::new(&settings::builder());
+        match verify_function(&func, &flags) {
             Ok(()) => {}
             Err(err) => panic!(err.message),
         }