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.

cranelift/src/filetest/simple_gvn.rs

@@ -40,7 +40,7 @@ impl SubTest for TestSimpleGVN {
 
         comp_ctx.flowgraph();
         comp_ctx.simple_gvn();
-        comp_ctx.verify(context.isa).map_err(|e| {
+        comp_ctx.verify(context.flags_or_isa()).map_err(|e| {
             pretty_error(&comp_ctx.func, context.isa, Into::into(e))
         })?;