DAG: Recognize no-signed-zeros-fp-math attribute

clang already emits this with -cl-no-signed-zeros, but codegen
doesn't do anything with it. Treat it like the other fast math
attributes, and change one place to use it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@293024 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed