[SROA] Fix crash due to bad bitcast

Summary:
As shown in the test case, SROA was crashing when trying to split
stores (to the alloca) of loads (from anywhere), because it assumed
the pointer operand to the loads and stores had to have the same
address space. This isn't the case. Make sure to use the correct
pointer type for both the load and the store.

Reviewed By: yaxunl
Differential Revision: https://reviews.llvm.org/D32593

llvm-svn: 304585