[WebAssembly] Don't use set_local instructions explicitly.

[WebAssembly] Don't use set_local instructions explicitly.

The current approach to using get_local and set_local is to use them
implicitly, as register uses and defs. Introduce new copy instructions
which are themselves no-ops except for the get_local and set_local
that they imply, so that we use get_local and set_local consistently.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@253905 91177308-0d34-0410-b5e6-96231b3b80d8