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
Chris Bieneman