[AMDGPU] Update `mad-mix*` CodeGen tests

- Use `fneg %a` instead of `fsub -0.0, %a`
  - This is for D134354 as we don't currently support folding `fsub -0.0, %a` into `fneg` on GISel.
    Also, `fneg` is the canonical way to do the negation.
- Switch to `update_llc_test_checks`-generated tests.
  - Better test coverage
  - Easier to update
  - Easier to see changes in future diffs
- Remove unnecessary CL arguments in RUN lines

Motive for the patch: Preparation for D134354 - we would like to
put GISel tests in this file as well. Fixing the lack of `fneg` and
switching to generated testing makes it much easier.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D134793