[llvm] [GISel] Add support for scalable vectors in getLCMType (PR #80306)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

This looks odd. In your examples, you try to unmerge a scalable vector. In how many register do want to unmerge  the scalable vector. From the documentation:

```
%bits_0_7:(s8), %bits_8_15:(s8),
    %bits_16_23:(s8), %bits_24_31:(s8) = G_UNMERGE_VALUES %0:(s32)
```
We unmerge 32 bits into 4 registers of 8 bits. For scalable  vectors, the number of output registers is unknown.

https://github.com/llvm/llvm-project/pull/80306