[WebAssembly] Implement multivalue call_indirects
Unlike normal calls, call_indirects have immediate arguments that
caused a MachineVerifier failure without a small tweak to loosen the
verifier's requirements for variadicOpsAreDefs instructions.
One nice thing about the new call_indirects is that they do not need
to participate in the PCALL_INDIRECT mechanism because their post-isel
hook handles moving the function pointer argument and adding the flags
and typeindex arguments itself.
Subscribers: dschuff, sbc100, jgravelle-google, hiraditya, sunfish, llvm-commits
Differential Revision: https://reviews.llvm.org/D74191