[SimplifyInst] Use correct type for GEPs with vector indices.

The current code does not properly handle vector indices unless they are
the first index.

At the moment LangRef gives the impression that the vector index must be
the one and only index (https://llvm.org/docs/LangRef.html#getelementptr-instruction).

But vector indices can appear at any position and according to the
verifier there may be multiple vector indices. If that's the case, the
number of elements must match.

This patch updates SimplifyGEPInst to properly handle those additional
cases.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D99961