[llvm] [AMDGPU] Merge two V_CNDMASK instructions into V_DUAL_CNDMASK (PR #135007)

[Ana Mihajlovic via llvm-commits]

mihajlovicana wrote:

> > for using vop2 instead of vop3 format
> 
> Normally this kind of shrinking would be done in SIShrinkInstructions. Is there a reason you can't do it there?

I wasn't sure if that would be proper because I am changing not only v_cndmask but also cmp instructions.

https://github.com/llvm/llvm-project/pull/135007