[llvm] [SystemZ] Add a SystemZ specific pre-RA scheduling strategy. (PR #135076)

[Jonas Paulsson via llvm-commits]

JonPsson1 wrote:

Patch rebased.

- Added an option for experimentation with latency reduction (-prera-lat-red). The default is the same as before, but there is also always/more/never/generic.

- LBM has now fluctuated back to being slower on main, so the regression handling on that has now become an improvement against main. The older version that had latency reduction enabled more and showed the regression is now equal with main. Imagick has likewise also reverted on main to being slower, while remaining faster with patch.

- Updated tests so that they are now passing also with latest changes.

https://github.com/llvm/llvm-project/pull/135076