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.

llvm-svn: 13415