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

[WÁNG Xuěruì via llvm-commits]

https://github.com/xen0n commented:

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.

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