[NVPTX] Let there be One True Way to set NVVMReflect params.

Summary:
Previously there were three ways to inform the NVVMReflect pass whether
you wanted to flush denormals to zero:

  * An LLVM command-line option
  * Parameters to the NVVMReflect constructor
  * Metadata on the module itself.

This change removes the first two, leaving only the third.

The motivation for this change, aside from simplifying things, is that
we want LLVM to be aware of whether it's operating in FTZ mode, so other
passes can use this information.  Ideally we'd have a target-generic
piece of metadata on the module.  This change moves us in that
direction.

Reviewers: tra

Subscribers: jholewinski, llvm-commits

Differential Revision: https://reviews.llvm.org/D28700

llvm-svn: 292068