[WebAssembly] Simplify regcopy.mir

Summary:
This deletes the ll templates from the functions because they don't need
them (mir files need ll templates only when they have function calls or
BB names that are not numbers).

This also renames the filename to `reg-copy.mir`, because I'm planning
to add some more `reg-*.mir` soon.

Reviewers: tlively

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64704

llvm-svn: 366140