[llvm] [LoongArch] Add custom lowering for BCOND and perform BR_CC combine (PR #147885)

[via llvm-commits]

heiher wrote:

> Is it possible to split this optimization into several small parts? For example:
> 
> * lowerBRCOND
> * performBR_CCCombine
> * performSELECT_CCCombine

Sure. We can split it into [`lowerBRCOND`, `performBR_CCCombine`] and [`performSELECT_CCCombine`].

https://github.com/llvm/llvm-project/pull/147885