[clang] [SveEmitter] Explicitly merge with `poison` instead of `undef` (PR #124596)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

Does the Arm spec actually allow us to do this?  It says "the inactive elements of the result have arbitrary, undefined values."  That seems very different from "your program has undefined behavior if you use the inactive elements of the result".  For comparison, the x86 `_mm_undefined_ps()` is lowered zeroinitializer, based on similar language.  Although, we didn't have `freeze poison` at the time, so that's also a possibility.  See #31524.

https://github.com/llvm/llvm-project/pull/124596