[WebAssembly] MC: Remove unused code for handling of wasm globals

For now, we are not using wasm globals, except for modeling of
the stack points.

Alos, factor out common struct WasmGlobalType, which matches the
name for that tuple in the Wasm spec and rename methods
to "isBindingGlobal", "isTypeGlobal" to avoid ambiguity.

Patch by Nicholas Wilson!

Differential Revision: https://reviews.llvm.org/D42750

llvm-svn: 323901