[WebAssembly] Add AsmString strings for most instructions.

[WebAssembly] Add AsmString strings for most instructions.

Mangling type information into MachineInstr opcode names was a temporary
measure, and it's starting to get hairy. At the same time, the MC instruction
printer wants to use AsmString strings for printing. This patch takes the
first step, starting the process of adding AsmStrings for instructions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@252203 91177308-0d34-0410-b5e6-96231b3b80d8