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/runone.rs

@@ -119,9 +119,11 @@ fn run_one_test<'a>(
 
     // Should we run the verifier before this test?
     if !context.verified && test.needs_verifier() {
-        verify_function(&func, isa).map_err(|e| {
-            pretty_verifier_error(&func, isa, e)
-        })?;
+        verify_function(&func, context.flags_or_isa()).map_err(
+            |e| {
+                pretty_verifier_error(&func, isa, e)
+            },
+        )?;
         context.verified = true;
     }