[llvm] [GlobalIsel][NFC] Fix LLT Propagation (PR #119587)

[Amara Emerson via llvm-commits]

aemerson wrote:

I'm wondering if we should add a new API so that it's convenient to create a new LLT based on the existing vector LLT. Instead of having to manually call `getElementType()`. Maybe `LLT::fixed_vector(unsigned EC, LLT &OrigLLT)` that pulls the type from the original one?

https://github.com/llvm/llvm-project/pull/119587