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

[Thorsten Schütt via llvm-commits]

tschuett wrote:

i12
```
entry:
  %s0s = zext <16 x i12> %s0 to <16 x i32>
  %s1s = zext <16 x i12> %s1 to <16 x i32>
  %m = add <16 x i32> %s0s, %s1s
  ret <16 x i32> %m
}
```
We pushed the `zext` through the vector and we ended with `<16 x i12>`vectors to legalize.


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