Remove reserved_reg functionality. (#424)

* Remove reserved_reg functionality.

This wasn't implemented, and if we need it in the future, it seems like
it would be better to extend the concept of global values to cover this.

* Use GlobalValue::reserved_value() for sentinal values.

lib/wasm/src/environ/dummy.rs

@@ -172,7 +172,7 @@ impl<'dummy_environment> FuncEnvironment for DummyFuncEnvironment<'dummy_environ
         let gv = func.create_global_value(ir::GlobalValueData::VMContext { offset: 0.into() });
 
         func.create_heap(ir::HeapData {
-            base: ir::HeapBase::GlobalValue(gv),
+            base: gv,
             min_size: 0.into(),
             guard_size: 0x8000_0000.into(),
             style: ir::HeapStyle::Static {