[WebAssembly] Switch to MC for instruction printing.

This encompasses several changes which are all interconnected:
 - Use the MC framework for printing almost all instructions.
 - AsmStrings are now live.
 - This introduces an indirection between LLVM vregs and WebAssembly registers,
   and a new pass, WebAssemblyRegNumbering, for computing a basic the mapping.
   This addresses some basic issues with argument registers and unused registers.
 - The way ARGUMENT instructions are handled no longer generates redundant
   get_local+set_local for every argument.

This also changes the assembly syntax somewhat; most notably, MC's printing
use sigils on label names, so those are no longer present, and push/pop now
have a sigil to keep them unambiguous.

The usage of set_local/get_local/$push/$pop will continue to evolve
significantly. This patch is just one step of a larger change.

llvm-svn: 252858