[llvm] [AMDGPU][True16][Codegen] remove another build_vector pattern from true16 (PR #149861)

[Brox Chen via llvm-commits]

broxigarchen wrote:

> Its not obvious from the test changes why this was illegal. May need a specific test that fails to compile without this patch.

This patch aims to fix a functional failure so it might be a bit difficult to find a ll test which shows the functional error (could be a large test). I'll add a mir test just like in https://github.com/llvm/llvm-project/pull/153143

https://github.com/llvm/llvm-project/pull/149861