This adds missing utility methods and copy instruction handling for
exnref type and also adds tests.
tee instruction tests are missing because isTee is currently only
used in ExplicitLocals pass and testing that pass in mir requires
serialization of stackified registers in mir files, which is a bit
nontrivial because MachineFunctionInfo only has info of vreg numbers
(which are large integers) but not the mir's register numbers. But this
change is quite trivial anyway.