[WebAssembly] Add mem.* intrinsics.

The grow_memory and current_memory instructions are expected to be
officially renamed to mem.grow and mem.size. Introduce new intrinsics
with the new names. These new names aren't yet official, so for now,
use them at your own risk.

Also, take this opportunity to add arguments for the currently unused
immediate field in those instructions.

llvm-svn: 323222