Add DXILPrepare CodeGen pass

The DXIL Prepare pass handles the IR mutations required to convert
modern LLVM IR into something that more closely resembles LLVM-3.7 IR
so that the DXIL bitcode writer can emit 3.7 IR.

This change adds the codegen pass handling the first two IR
transformations:

* stripping new function attributes
* converting fneg into fsub

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D122081