Check in the experimental GEP splitter pass. This pass splits complex

Check in the experimental GEP splitter pass. This pass splits complex
GEPs (more than one non-zero index) into simple GEPs (at most one
non-zero index).  In some simple experiments using this it's not
uncommon to see 3% overall code size wins, because it exposes
redundancies that can be eliminated, however it's tricky to use
because instcombine aggressively undoes the work that this pass does.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@85144 91177308-0d34-0410-b5e6-96231b3b80d8