protected (some) complex variants from left-trimming in normalize

[selkovjr]

This patch prevents the trimming of complex variants that are already normalized.

Example: trimming CAT -> CG changes it to a form that is hard to analyze: AT -> G. Indeed, some tools, such as freebayes, interpret it incorrectly, while the original combination of a deletion and a SNP presents no problems.

I do not know whether the solution I present here is complete (I suspect not), but it fixes all instances of this type of error I was able to find in my data.

[atks]

@selkovjr - I cannot accept this PR because:

1. it would contradict the definition of normalization
2. it is not practical to make exceptions so that a downstream tool will work
3. It is probably a better idea for you to add an intermediate step between vt normalize and freebayes to manipulate the variants