Disable phi-of-op optimization in NewGVN
Disable phi-of-op optimization in NewGVN

The phi-of-op optimization is buggy. Although there are recently some new
patches by Florian to fix outstanding issues (D66924, D99987), the issue
that block us is still not fixed (https://bugs.llvm.org/show_bug.cgi?id=49873),
I haven't come up with a proper fix for my reported issue, and I believe
there may be more underlying issue in the dependency tracking part for phi-of-ops
optimization. I think we need a long way to make the algorithm sound.
I would like to disable this specific optimization to avoid miscompile unkown
user program. I will keep on eye on the NewGVN progress and will turn on
when I believe this specific optimization is mature.

We original switch to NewGVN to mainly fix (https://github.com/GPUOpen-Drivers/llpc/issues/957)
I have tested the NewGVN can still optimize off this pattern with this
patch.

googlestadia/llpc,googlestadia/llpc,GPUOpen-Drivers/llpc,googlestadia/llpc,GPUOpen-Drivers/llpc,googlestadia/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,googlestadia/llpc,GPUOpen-Drivers/llpc