[llvm] [LoongArch] Emit function call code sequence as `PCADDU18I+JIRL` in medium code model (PR #76555)

[via llvm-commits]

wangleiat wrote:

> Sorry for the late review and happy new year!
> 
> It seems the commit diff is a bit large though -- there's a whole refactoring (separating large code model logic into several different functions, plus code movement) being lumped together with the `call36` implementation. It could be easier to split into two PRs so each change stays focused. I've lightly checked the `call36` test case changes and they look good though.

Thank you very much for your suggestion. I have already implemented the split as you advised.

https://github.com/llvm/llvm-project/pull/76555