[llvm] [AMDGPU][SIPreEmitPeephole] pre-commit tests: mustRetainExeczBranch: use a cost model (PR #109816)

[Juan Manuel Martinez Caamaño via llvm-commits]

jmmartinez wrote:

Only look a the last commit.

PR for the first commits in [pre-commit tests](https://github.com/llvm/llvm-project/pull/109812) and [implementation](https://github.com/llvm/llvm-project/pull/109813)

https://github.com/llvm/llvm-project/pull/109816