Add an option to GVN to remove all partially redundant loads. This is currently

Add an option to GVN to remove all partially redundant loads.  This is currently
disabled by default.  This divides the existing load PRE code into 2 phases:
first it checks that it is safe to move the load to each of the predecessors
where it is unavailable, and then if it is safe, the code is changed to move
the load.  Radar 7571861.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@95007 91177308-0d34-0410-b5e6-96231b3b80d8