[llvm] [LowerMemIntrinsics] Factor control flow generation out of the memcpy lowering (PR #169039)

[Alexey Bataev via llvm-commits]

alexey-bataev wrote:

Should this be enabled at O0 at all? We see code size/runtime increase at O0 with the debug info enabled because of this patch (about 3x runtime perf regressions, code size bloat about 1.5x)

https://github.com/llvm/llvm-project/pull/169039