Revert "[DAGCombiner] Split up an indexed load if only the base pointer value is live"

This reverts r208640 (I've just XFAILed the test) because it broke ppc64/Linux
self-hosting. Because nearly every regression test triggers a segfault, I hope
this will be easy to fix.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209747 91177308-0d34-0410-b5e6-96231b3b80d8