[SROA] Support opaque pointers

Make the following changes in order to support opaque pointers in SROA:

 * Generate i8 GEPs for opaque pointers.
 * Explicitly enforce that promotable allocas only have stores of
   the alloca type -- previously this was implicitly enforced.
 * Replace a check for pointer element type with load/store type.

Differential Revision: https://reviews.llvm.org/D109259