[llvm] Scalarizer : Fix vector shuffle issue when can't aligned to customized minBits. (PR #163912)

[via llvm-commits]

ShchchowAMD wrote:

> Oh, I understand the problem now!
> 
> A much easier fix is to clear out the `ExtendMask` so that everything past `NumPacked` elements is -1.
> 
> And reminder to replace the second operand of the extending shuffle with a poison value to generate the canonical form directly.

Oh, yes, that's much better.

I've updated current implementation and the test, now it will only truncate and resize extend mask to let last fragment match requirement of shuffleVector.

https://github.com/llvm/llvm-project/pull/163912