Reapply r221924: "[GVN] Perform Scalar PRE on gep indices that feed loads before

Reapply r221924: "[GVN] Perform Scalar PRE on gep indices that feed loads before
doing Load PRE"

This commit updates the failing test in
Analysis/TypeBasedAliasAnalysis/gvn-nonlocal-type-mismatch.ll

The failing test is sensitive to the order in which we process loads.  This
version turns on the RPO traversal instead of the while DT traversal in GVN.
The new test code is functionally same just the order of loads that are
eliminated is swapped.

This new version also fixes an issue where GVN splits a critical edge and
potentially invalidate the RPO/DT iterator.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222039 91177308-0d34-0410-b5e6-96231b3b80d8