[GVN] Perform Scalar PRE on gep indices that feed loads before doing Load PRE.

Phabricator Revision: http://reviews.llvm.org/D6103
Patch by "Balaram Makam" <bmakam@codeaurora.org>!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221924 91177308-0d34-0410-b5e6-96231b3b80d8