[SROA] Use offset sizes from the DataLayout instead of the pointer siezes.

This fixes an assertion when constant folding a GEP when the part of the offset
was in i32 (IndexSize, as per DataLayout) and part in the i64 (PointerSize) in
the newly created test case.

Differential Revision: https://reviews.llvm.org/D52609

llvm-svn: 345585