[WebAssembly] Add WebAssemblyMCInstLower.cpp.

This isn't used yet; it's just a start towards eventually using MC to
do instruction printing, and eventually binary encoding.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@252194 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed