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

[Thorsten Schütt via llvm-commits]

tschuett wrote:

For reference, we did the same combine/pattern with selects. It was neutral: https://github.com/llvm/llvm-project/pull/100539.

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