This diff adds a pass to the arith dialect that will set the fastmath
attribute for all operations that support the ArithFastMathInterface
interface. This can be useful to apply module-level fastmath settings in a way
that is similar to setting LLVM fastmath values for an entire file.
Future pass modifications may include adding an option to perform a bitwise
operation on the fastmath flags, to (for example) combine the pass argument with
an existing fastmath value with a bitwise or.