[ConstraintElimination] Do not crash on vector GEP in decomposeGEP

Commit 359bc5c5 caused
 Assertion `isa<To>(Val) && "cast<Ty>() argument of incompatible type!"'
failures in decomposeGEP when the GEP pointer operand is a vector.

Fix is to use DataLayout::getIndexTypeSizeInBits when fetching the
index size, as it will use the scalar type in case of a ptr vector.

Differential Revision: https://reviews.llvm.org/D137185