[ConstantFold] Support vector index when factoring out GEP index into preceding dimen...

[ConstantFold] Support vector index when factoring out GEP index into preceding dimensions

Follow-up of r316824. This patch supports the vector type for both current and
previous index when factoring out the current one into the previous one.

Differential Revision: https://reviews.llvm.org/D39556

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@319683 91177308-0d34-0410-b5e6-96231b3b80d8