Implement folding of GEP's like:

Implement folding of GEP's like:

        %tmp.0 = getelementptr [50 x sbyte]* %ar, uint 0, int 5         ; <sbyte*> [#uses=2]
        %tmp.7 = getelementptr sbyte* %tmp.0, int 8             ; <sbyte*> [#uses=1]

together.  This patch actually allows us to simplify and generalize the code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@13415 91177308-0d34-0410-b5e6-96231b3b80d8