[llvm] [WebAssembly] Split separate component LiveIntervals for TEEs (PR #131561)

[Heejin Ahn via llvm-commits]

aheejin wrote:

@qcolombet Thanks for the info; I'll take a look. Meanwhile, do you know why this verification rule, which requires a `LiveIntervals` should not contain unconnected components, exist?
https://github.com/llvm/llvm-project/blob/f4043f451d0e8c30c8a9826ce87a6e76f3ace468/llvm/lib/CodeGen/MachineVerifier.cpp#L3923-L3936

I haven't read the implementation of https://github.com/llvm/llvm-project/blob/main/llvm/lib/CodeGen/RegisterCoalescer.cpp, but in general what register coalescing does is to reuse registers if live ranges don't overlap, which I think naturally create multiple unconnected components. For example,
```
0  %a = ...
1  ... use %a ...  ;; %a is dead after here
2  %b = 
3  ... use %b ...
```
In this pseudocode, when `%a` and `%b` are registers, if the types of `%a` and `%b` are the same and their live ranges don't overlap, register coalescing can make the code only use `%a`:
```
0  %a = ...
1  ... use %a ...
2  %a = 
3  ... use %a ...
```
But this can create unconnected live intervals for %a, like `[0r,1r), [2r,3r)`.

https://github.com/llvm/llvm-project/pull/131561