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/filetests/verifier/return_at_end.cton

@@ -1,10 +1,6 @@
 test verifier
 set return_at_end
 
-; The verifier doesn't have an API for verifying with flags without also
-; setting an ISA.
-isa riscv
-
 function %ok(i32) {
 ebb0(v0: i32):
     brnz v0, ebb1