[WebAssembly] Implement ref.is_null MC layer support and codegen

Custom type-checking (in WebAssemblyAsmTypeCheck.cpp) is used to
workaround the fact that separate variants of the instruction are
defined for externref and funcref.

Based on an initial patch by Paulo Matos <pmatos@igalia.com>.

Differential Revision: https://reviews.llvm.org/D123484