[WebAssembly] Use a physical register to describe ARGUMENT liveness.

Instead of trying to move ARGUMENT instructions back up to the top after
they've been scheduled or sunk down, use a fake physical register to
create a liveness constraint that prevents ARGUMENT instructions from
moving down in the first place. This is still not entirely ideal, however
it is more robust than letting them move and moving them back.

llvm-svn: 254084