[x86] add tests to show potential opt-out of ftrunc optimization; NFC

This is another preliminary step for disabling this transform as 
discussed in the post-commit thread for:
rL330437
I'm using one of the names suggested there for the attribute, but 
we can fix that up as needed once the clang side of this is sorted 
out. 


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@330950 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed