[SROA] Fix APInt size when load/store have different address space

Currently there is a bug in SROA::presplitLoadsAndStores which causes assertion in
GEPOperator::accumulateConstantOffset.

Basically it does not consider the situation that the pointer operand of load or store
may be in a non-zero address space and its size may be different from the size of
a pointer in address space 0.

This patch fixes assertion when compiling Blender Cycles kernels for amdgpu backend.

Diffferential Revision: https://reviews.llvm.org/D33298

llvm-svn: 305107