[llvm] [AArch64] Tweak truncate costs for some scalable vector types (PR #119542)

[David Sherwood via llvm-commits]

david-arm wrote:

Ignoring the `undef deprecator found issues in your code.` message for this PR, since a) I'm modifying and extending existing tests that already use `undef` and b) I don't know how critical it is to avoid the use of `undef` for cost model tests since the input values are irrelevant here.

https://github.com/llvm/llvm-project/pull/119542