VM: Double switch statement speed by not emitting the ranges dispatch if there are no range cases

[schungx]

A very simple optimization for the switch statement.

Currently, a single switch statement lowers into two calls to Op::Switch -- one for the cases and one for the ranges.

In the predominant scenario of no range cases, there is no need to emit the dispatch to empty ranges.

This PR optimizes the lowering such that no second dispatch is emitted if there are no range cases. It also saves the push and pop of the Scope by not storing the switch value.

Speed of the switch statement doubles to close to 2x the speed of the AST walker when there are no range cases.

[schungx]

@ImTheSquid I have re-architect the PR. The push to scope is removed completely. The scope is no longer touched.

Instead, the Switch op is modified so that it does not pop the value, only inspects it. This is done to allow a subsequent Switch op call to also inspect eh value.

The compiler only emits a pop at the end of the switch statement running to completion.

The only tricky aspect is that those "stack surpluses" must also be popped if there is a break statement terminating an outer loop prematurely. So I keep a count of how many of these surpluses exist.