Simplify scalable vector testing

[sayantn]

For SVE types, e.g svint8_t, we can convert &svint8_t safely to &[i8] with length svcntb().
For svbool_t, it is equivalent to <vscale x 16 x i1>, so the layout is the same as an i8 array of size 2*vscale, so we can use svcntd() to convert &svbool_t to &[i8]. I used a small repr(transparent) struct with a custom Debug implementation to make the debug output nicer

This approach seems to work nicely, at least for now (it was indeed failing a few days ago, maybe some LLVM update fixed it recently)

r? @davidtwco

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.