[VM] add removeUnusedFunctions pass in vm memoryopt

[zxy844288792]

If unused function presents like a cond branch never been accessed, memory allocation pass will throw erros since it is not presented in context_analysis map like errors shows below:

TVMError: Check failed: (it != context_analysis_map_.end()) is false: Cannot find expr in the context analysis map:
E           #[version = "0.0.5"]
E           free_var %x2: Tensor[(2, 2), float32];
E           %0 = fn (%p0: Tensor[(2, 2), float32], Primitive=1) -> Tensor[(2, 2), float32] {
E             multiply(%p0, 3f /* ty=float32 */) /* ty=Tensor[(2, 2), float32] */
E           };
E           %0(%x2) /* ty=Tensor[(2, 2), float32] */

Add RemoveUnusedFunctions pass before memory allocation to avoid this issue.

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[comaniac]

@jroesch @icemelon9 @zhiics could you guys help review this PR?