[llvm] [LoongArch] Implement getJumpTableIndex to allow splitting jump table edges (PR #228550)

[via llvm-commits]

lrzlin wrote:

> Why doesn't LoongArch need [`isJumpTableLoad`](https://github.com/llvm/llvm-project/pull/224197/changes#diff-dc083bdfb8b95291a8e5e635119a632c192995b3d371ee671c4d507dddc29accR1859-R1860)?

For LA64's `LDX_*`, we've checked them in `MO.isJTI()`, but this is needed for LA32's `LD_W`. 

https://github.com/llvm/llvm-project/pull/228550