[Tests] More unordered atomic lowering tests

This time, focused around narrowing and widening transformations.  Also, include a few simple memory optimization tests to highlight missed oppurtunities.  This is part of building up the test base for D57601.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353972 91177308-0d34-0410-b5e6-96231b3b80d8