[llvm] [GlobalISel] Combine G_UNMERGE_VALUES with anyext and build vector (PR #112370)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

```
 %and = and i48 %y, 15
  %ashr = ashr i48 %x, %and
  %trunc = trunc i48 %ashr to i16
  ret i16 %trunc
```
I bet `i48` is not a legal RISC-V type and the unmerge_values combine caused issues.

https://github.com/llvm/llvm-project/pull/112370