[llvm] [CodeGen][LLT] Add isFixedVector and isScalableVector (PR #71713)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> The current isScalable function requires a user to call isVector before hand in order to avoid an assertion failure in the case that the LLT is not a vector.

I'd consider that a bug in the first place. It shouldn't assert 

https://github.com/llvm/llvm-project/pull/71713