Reapply r298620: [LV] Vectorize GEPs

Reapply r298620: [LV] Vectorize GEPs

This patch reapplies r298620. The original patch was reverted because of two
issues. First, the patch exposed a bug in InstCombine that caused the Chromium
builds to fail (PR32414). This issue was fixed in r299017. Second, the patch
introduced a bug in the vectorizer's scalars analysis that caused test suite
builds to fail on SystemZ. The scalars analysis was too aggressive and marked a
memory instruction scalar, even though it was going to be vectorized. This
issue has been fixed in the current patch and several new test cases for the
scalars analysis have been added.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@299770 91177308-0d34-0410-b5e6-96231b3b80d8