[llvm] [GlobalIsel] Pust cast through build vector (PR #104634)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

I got 
```
LLVM ERROR: unable to legalize instruction: %0:_(<16 x s12>) = G_BUILD_VECTOR %67:_(s12), %68:_(s12), %69:_(s12), %70:_(s12), %71:_(s12), %72:_(s12), %73:_(s12), %74:_(s12), %75:_(s12), %76:_(s12), %77:_(s12), %78:_(s12), %79:_(s12), %80:_(s12), %81:_(s12), %82:_(s12) (in function: i12)
```
for neon-extadd.ll

Note the s12 vector elements.

https://github.com/llvm/llvm-project/pull/104634