implement support for sinking a load out the bottom of a block that

implement support for sinking a load out the bottom of a block that
has no stores between the load and the end of block.  This works
great and sinks hundreds of stores, but we can't turn it on because
machineinstrs don't have volatility information and we don't want to
sink volatile stores :(

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@45894 91177308-0d34-0410-b5e6-96231b3b80d8