Holding my nose and moving the accumulation routine to GEPOperator

Holding my nose and moving the accumulation routine to GEPOperator
instead of the instruction. I've left a forwarding wrapper for the
instruction so users with the instruction don't need to create
a GEPOperator themselves.

This lets us remove the copy of this code in instsimplify.

I've looked at most of the other copies of similar code, and this is the
only one I've found that is actually exactly the same. The one in
InlineCost is very close, but it requires re-mapping non-constant
indices through the cost analysis value simplification map. I could add
direct support for this to the generic routine, but it seems overly
specific.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@169853 91177308-0d34-0410-b5e6-96231b3b80d8