[vector] simple clean ups

[Yang Zhang (Arm Technology China)]

Hi Paul

Please check the following. 

>Observations:

>- the API is the same and yet not the same :-) the overall design has not changed but there has been some good compression of operations and better abstractions around conversion, the slice is a nice 
>abstraction from which shift/rotate elements can be expressed without resorting to a shuffle.
>
> - the shuffleTest is failing for certain vectors on my Mac [*] (regardless of my change set).  Is this a known issue?

Yes, it's a known issue. The root cause is described as follows:
In library_call.cpp, there is a node VectorLoadShuffle for generating shuffle vector. This node is used in both shuffleToVector and rearrange. But In x86 backend, this node is implemented with SIMD only for rearrange, not for shuffleToVector. To fix this issue, x86 backend needs to be updated.

Regards
Yang