Update my SROA changes in response to review.

* change FindElementAndOffset to return a uint64_t instead of unsigned, and
  to identify the type to be used for that result in a GEP instruction.
* move "isa<ConstantInt>" to be first in conditional.
* replace some dyn_casts with casts.
* add a comment about handling mem intrinsics.

llvm-svn: 91762