[x86] add tests for potential negation folds

These are the backend equivalents for the tests added in r284627.
The patterns may emerge late, so we should have folds for these in the DAG too.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@284842 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed