Remove the logic for reasoning about NaNs from the code that forms

Remove the logic for reasoning about NaNs from the code that forms
SSE min and max instructions. The real thing this code needs to be
concerned about is negative zero.

Update the sse-minmax.ll test accordingly, and add tests for
-enable-unsafe-fp-math mode as well.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@96775 91177308-0d34-0410-b5e6-96231b3b80d8