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

[Lu Weining via llvm-commits]

https://github.com/SixWeining approved this pull request.

LGTM. These 3 commits should have to be merged one by one locally. And the 2nd one's title ("xxx") should be updated.

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