[SVE] Change definition of reduction ISD nodes to have an SVE vector result type.

[SVE] Change definition of reduction ISD nodes to have an SVE vector result type.

The current nodes, AArch64::SMAXV_PRED for example, are defined to
return a NEON vector result.  This is incorrect because they modify
the complete SVE register and are thus changed to represent such.

This patch also adds nodes for UADDV_PRED and SADDV_PRED, which
unifies the handling of all SVE reductions.

NOTE: Floating-point reductions are already implemented correctly,
so this patch is essentially making everything consistent with those.

Differential Revision: https://reviews.llvm.org/D87843