[BranchFolding] Merge MMOs during tail merge

Summary:
If we remove the MMOs from Load/Store instructions,
they are treated as volatile. This makes other optimization passes unhappy.
eg. Load/Store Optimization

So, it looks better to merge, not remove.

Reviewers: gberry, mcrosier

Subscribers: gberry, llvm-commits

Differential Revision: http://reviews.llvm.org/D14797

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254694 91177308-0d34-0410-b5e6-96231b3b80d8