[WebAssembly] Add missing utility methods for exnref type

Summary:
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.

Reviewers: tlively

Subscribers: dschuff, sbc100, jgravelle-google, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64705

llvm-svn: 366149