[ConstantFolding] Fix vector GEPs harder

[ConstantFolding] Fix vector GEPs harder

For vector GEPs, CastGEPIndices can end up in an infinite recursion, because
we compare the vector type to the scalar pointer type, find them different,
and then try to cast a type to itself.

Differential Revision: https://reviews.llvm.org/D28009

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@290260 91177308-0d34-0410-b5e6-96231b3b80d8